How do you guys keep a game feeling grounded and the characters relatable and fragile at higher levels? This is easy up to level 5, but once things hit there things lose that feeling of grittiness. How do you guys avoid or deal with this?

Though I'm not a DM, I would personally make "encounters" less about physical conflict and more about emotional and moral conflict. What makes each character tick? What would they absolutely never do? What would happen if they were forced to do exactly that? Let's take a barbarian as an example.

The barbarian swore from an early age never to attack with an intent to torture. He would only ever attack to bring a swift death.

However, his target in this specific encounter tortured his loved one by putting them through excruciating agony to the point of death, then resurrecting them and repeating the process over and over, while forcing the barbarian to watch.

In a fit of uncontrollable, unbridled fury and grief, the barbarian lashes out at the torturer and enacts his own torture upon them, to make them feel what he felt. He hacks off all their limbs and plucks out their eyes and tongue and beats them to death with their own arm.

When he snaps out of this fury, he realises what he's done, and that his loved one was still there and watching him lose his mind.

What would happen next? That's for the player to explore.

 I am both a DM and a player, so keeping characters fragile and relatable is a subject that comes up a lot in my games. The group I play with happens to not like gritty and dark themes in their sessions, so it's far more difficult to enact moral and emotional conflict. However, I can say that you can make things challenging with a masterful use of an NPC/monster.

If an NPC your party befriends and loves is held captive or is arrested, you can put the party through Avernus and back. For example, if the NPC is held in a dark tower with traps and loads of high-level monsters, they would have to question whether or not they want to risk their life for him/her. 

If your party doesn't love an NPC or your playing in a published campaign, you can use the already included weak/boring monsters and spin them on their heads. An example would be the mercenary/bandit 'monsters'. If your party is ambushed in a tavern or inn (or want to ambush a bandit camp), wouldn't it be exciting for their enemies to be as cunning as they are? If the players flip a table/wagon, why shouldn't the enemy? Most of the time, players (at least mine) won't bother with resistance potions or magic armor, so why not have the bandits armed with Molotov cocktails and poison darts? You could also use traps, puzzles, mysteries, etc., to keep your players on their toes. 

These tips are how I keep my games interesting, and the characters relatable and fragile at higher levels. These ideas might not work for everyone, so if you get stumped in certain situations, think: " what would a real Monster/NPC do when its cornered/attacked?" or "How dangerous is the Monster/NPC, and how could I make them deadlier?" that should make the games more interesting. if you're blessed with talented role players, then this should all come naturally. :)

Really depends on what you mean by "fragile" and "grittiness." If you want characters to feel afraid for their lives and relatively at the mercy of your world, then yeah you need to keep it low level. The built-in progression of the game empowers the characters and eventually turns them into superheroes. I suppose you could prolong things a bit by limiting it to martial characters, but at that point, you're trying to put a square peg in a round hole. There are other games that do it better.

For grounded and relatable, you can look at movies and books with powerful characters. They still have weaknesses - the most common being the ones they love. Make some bonds in those first 5 levels and then threaten them later on. You could also really lean into the flaws of the characters, but that's going to require some additional player buy-in. You can't just say, "you spend all night drinking so when the attack comes at dawn you are not ready for it." But you can encourage the player to put themselves in those kinds of situations.

Im looking for grounded and relatable, but also not feeling like they can just go off and quest or adventure unprepared. In short I want to avoid them becoming the Avengers and be more like Agents of Shield in their first season or so. The things I'm planning on trying are after a certain level making HP increase only be equal to their con mod plus any feats that add to it, wizards don't automatically get new spells instead they have to research, other casters will have to spend time and train to explore and come into their new spells, that plus the gritty realism variant rules for rests in the DMG I'm hoping will keep things where more grounded. Thoughts?

It honestly depends on whether or not the players want to be Avengers; not worrying about monsters, hit points, spells, etc.. If they're ok with the game slowing down a bit and getting more challenging than easier, then that's a Great Idea!

Honestly, sometimes you just gotta give up. If you give up and let them control the story, it's a lot easier for them. You place the rails, and they conduct the train. If you move along, letting the players decide, it's easier for everyone. While I agree, yes, a moral conflict is a amazing way to spice up a campaign, it isn't necessarily the best. It depends on who you play with. Some people love hard combat. Some people love just hacking people to death, and some love a amazing story, even if the battles are trash.

If you think the campaign your in must be railroaded, then you should railroad, but make sure they have wiggle room. If the players want to kill that NPC, let them. Then replace the NPC in the future, but have the death come back to bite them.

I guess what I'm trying to say is : Sometimes you'll find yourself in need of some spice, sometimes you'll need some action. Feel free to throw in whatever you need, and move on like that.

Consequences, consequences, consequences. That will keep their feet firmly on the ground. Make them care about their characters and truly invest into them. Then make situations to bring them almost to the brink of death. It sure as heck worked for our party recently haha

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Im looking for grounded and relatable, but also not feeling like they can just go off and quest or adventure unprepared. In short I want to avoid them becoming the Avengers and be more like Agents of Shield in their first season or so. The things I'm planning on trying are after a certain level making HP increase only be equal to their con mod plus any feats that add to it, wizards don't automatically get new spells instead they have to research, other casters will have to spend time and train to explore and come into their new spells, that plus the gritty realism variant rules for rests in the DMG I'm hoping will keep things where more grounded. Thoughts?

 My issues with hacks like these is that they never apply equally. Imagine playing as a wizard in this game. You level up and you get no HP because you wanted to be good at skills and no spells. Meanwhile the Barbarian advances unhindered and the Cleric continues to have access to their entire spell list unless you want to houserule that too.

And what happens when you limit HP that much is you end up with encounters where little matters but the dice. If you roll well on the wizard, he will die in one hit. And then if you roll well on the cleric - boom, it's a TPK. Making battles that swingy really minimizes any tactical choices the characters can make in combat and reduces it to a crapshoot.

I'm much more in favor of making the overall environment more hostile and leaving character mechanics alone. Make travel difficult with extreme weather, roving enemies, limited food and water. Show the desperation in the world through other characters - have the party come across the remains of other groups that perished in a variety of disturbing ways. A lot of it is just establishing the atmosphere. This combined with the gritty realism rules in the DMG should be enough. Go too far beyond that and you're just punishing your players - winning despite adversity is really rewarding, but losing with feelings of hopelessness and futility can be really frustrating.

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How do you guys keep a game feeling grounded and the characters relatable and fragile at higher levels? This is easy up to level 5, but once things hit there things lose that feeling of grittiness. How do you guys avoid or deal with this?

Fundamentally, you don't.

The game is broken up into four tiers (1-4, 5-10, 11-16, 17+) for a reason. Curiously enough, D&D isn't the only game where these four tiers exist - I've seen it appear enough time that I suspect its part of a kind of game theory. 

Anyways, the first tier, aka the apprentice tier, you're basically dealing with events on a street level or small town. Things are dangerous, but doable. The first four levels are the ones where you're still learning your trade, its gritty, dangerous, and tricky. This is what you want to play, so the best way to do that is to effectively draw out the length of this time.

The second tier, starting at level 5, is where you start facing challenges that challenge whole city-states, small countries, and the like. You have the power to be seen as a champion of said city, rivaling the captain of the guard, nobles, and more. This is going to be less gritty by default, because the challenges you face require you to have the power and ability to affect major cities within only four people. 

Third tier is where we start dealing with the fate of political entities with the clout of the fantasy-equivalent of the United States, European Union, China.  Basically, four people have the power to affect entire continents and change the fate of an empire. 

Fourth tier, 17 through Epic, is global / inter-dimensional havoc. You treat with gods, basically. 

There's a reason that the overwhelming majority of games start to fail past level 10; the traditional D&D plot doesn't quite flow as nicely when you start getting tier 3 abilities, and the plots need to start acting on a near-global level, which isn't easy. I get the feeling, from what's been said here, that you enjoy the low level D&D, and want to keep at it. If so, I recommend checking out ECL 6 from D&D 3.x. They might have some ideas for you.

Part of the answer is not letting the players become tougher than anything around them all the time.  So, in my towns, the players become much tougher than your average towns guard, but there are still figures like the Captain of the Guard that they know they need to be mindful of because he can go toe-to-toe with the toughest player in the party.  Tough figures connected with society need to be present, or the party becomes "bigger" than the government, and this will create problems you shouldn't have.  Likewise, you can't make all the monsters as tough as the players, but the monsters need chiefs & shaman that will give the party a run for their money.  Not every encounter, but enough that they don't blow all their spells on a group of scouts, since the next encounter might have some tough opponents.  If the party simply takes a long rest every time they blow a bunch of spells, even when the encounter didn't warrant the use of so many spells, then there needs to be some correction.