Beginner gm seeking advice before my sanity goes out the window

Overly dramatic title aside, here is the situation.

First off a little kind of disclaimer. So far everyone has fun but at this point I realized, that most encounter, even bosses don´t impose any real threat to the party and I think it´s mostly my fault, so I try to find a way to ballance things out a bit.

I am not trying to tpk them or kill single charakters on pupose. I just want a bit more challenge. 

Currently running the crooked moon as my first campaing. because of this, I decided to stay as close as possible to the books descriptions. 

The Party consists of six players. clearly my first mistake (but sessions take place when at least 4 are available due to the Campaing being intended for a party of four and it shows in regards of how encounters play out)

Characters in the party are: bard, druid, barbarian, cleric, paladin, rogue. no multiclasses so far.

We are halfway throug the story so far and it get´s hard to even put a scratch on them

well... this party has a quite strong synergy (also the Campaign throws magic items left and right on them). Experiment barbarian uses his maddening fumes trait to force anemies to do a wis save or have disadvantage on every target except him. Twillight cleric has a AC of 24 due to a plate armor he stole (whoever thinks to do the campaing, looking back i strongly advise to change the part with the three knights in the crimson monastery stables to the knights wearing their armor, so you can claim that the armor got destroyed in battle...), the druids´s sanctum of the sheperd, the shield of Marius Renathyr (that grants him a whooping +4 AC) and the druids wicker effigy granting +1. also whenever the unlikely event happens that he get´s hit, he uses the shields reaction to just deny the hit (most common enemies have not that good + to hit.. so they barely ever hit him aside from crits oder close hits) he also has twillight sanctury active in most encounters, so the largest part of damage dealt to any partymember get´s blocked or undone by regained temp HP. 

difference shows whenever this player is not present in a session

Now I guess most people would advise to use abilities with saves, bringing me to the next part of the synergy. The party allways crowding around the paladin, grants them +5 on their saves. prime target for a fireball (or similar aoe), right? 

yeah well, none of the casters so far has fireball.. or dispell magic or counterspell. All the aoe stuff aside from insect swarm on a higher level in a location that would be highly disadvantageus or the caster, is recharge 5-6. Also damage get´s undone by either twillight sanctuary or the tons of healing at their disposal. simply throwing more minons at them? yeah, hypnotic pattern or spirit guardians... it´s kinda trivializing the encounters and everyything I try to ballance things out seem to only lengthen encounters.

Also whenever there is a situation where during travel or that´s not happening on a battlemap and any kind of saving throws is needed, the paladin claims to keep everyone in 10 ft. reach around him. (absolutely trivialized the Hartsblight forest chapert so far, where a dc 10 Con save is needed whenerver the party travels to the next location or they gain a stacking debuff) 

so that brings me to the question, what would be your advise on how to ballance things a bit towards tha party size and situation? 

Giving more HP to everything just makes encounters go longer so far. 

More enemies seems not to be very effective so far. Threw a improvised encounter at them that was labeled as deadly by a en****er calculator. Not a single scratch on them... aside from the druid getting dragged off and mauled by a bat hound. 

my goal is to spice up the challende a bit, giving them at least a greater feeling of being in danger 

My current idea would be to give caster enemies more spells like counterspell, dispell magic or heat metal. lowering recharge time? Turing some abilities into bonus actions?

What would be your advice?

Use terrain to split them up and give monsters some AoE and countermagic. A maze-like area forcing players into single file and then ranged attackers pummeling them from above. Make the maps difficult for them. Battles can be much more than just stats with the right terrain. Play enemies smart. Pounding them with AoE will teach them to spread out, which then allows you to engage them one on one, or a few on one, and makes the battles more interesting. Figure out how to turn their tactics against them.

I think the guidelines for encounter balance are garbage and produce cake walk combat.

I regularly thow dozens of monsters at my players and i usually knock one player unconscious. If your players have an aoe, you need  a mob of closely packed cannon fodder to.draw their fire.  Make them high emough to last 1.5 rounds, and the player casting fireball will feel awesome. Meanwhile have at least 2 decent front liners spread out and coming from 2 different angles to keep the party tank busy. Then you need a random monster for everyone else. Some range attack fodder to go after the players at thr back of the map.

Have the front liners focus fire on the tank until they go down, then disperse and diffuse them so their damage is speead out to.give the party time enough to heal the tank and kill the big bads.

The other thing where encounter balance formula drop the ball.is they dont scale for whether the encounter is the first after a long rest or the 6th encounter since a long rest.

If the party just had a long rest, expect the first fight to not challenge the party nearly as much.as the sixth fight.

Id the party just had a long rest, make sure the party runs into a couple random encounters or fights in the foyer before the main event. Otherwise the party will go nova on the bad guy and itll be iver pretty quick.

ok, so with your input in mind and after some research I did and some talks I had, I´ve come to the following conclusions.

Because Encoutners in this campaign are pre designed I don´t have much room, to edit them. also maps are fixed for these encoutners. But I regularly add more enemies etc. 

Whole parties Ac goes down by 1, because apparently the druid doesn´t know his spells (as a beginner I heavily rely on the players explaining their spells or sending them in vtt... a thing I won´t do aganin in the future) and lacks the component, he needs to cast "sanctum of the shepert" 

less long rests, more pressure.

I´ll stray from the "immersive" combat towards more of a optimised one. with the argumentation, that Creatures know what they are good at and also know to identify their preffered targets. 

talked to the cleric about the shileds reaction and that despite the wording, my interpretation is pretty clear about him not being able to use it on himself

also realized, that due to their fabled heirlooms, their save dc´s are pretty high for their level, why I´ll adjust enemy save dc´s. also because the paladin clumping issue. 

probably will adjust enemy caster spell lists ab bit (some are in desperate need of dispel magic and similar) 

I´ll try it like that and see how this weeks session plays out

A given CR should be a deadly challenge for one character of the same level, (almost) always. Also, the more actions a team can take in one round, the greater the advantage. You can take that as a base and do other things, like splitting the party, adding environmental hazards etc.

Also, if a character wants to don armor they've looted, as a DM you can just say that it doesn't fit and needs to be adjusted.

Quote from alvarojosemarquez >>

A given CR should be a deadly challenge for one character of the same level, (almost) always. Also, the more actions a team can take in one round, the greater the advantage. You can take that as a base and do other things, like splitting the party, adding environmental hazards etc.

Also, if a character wants to don armor they've looted, as a DM you can just say that it doesn't fit and needs to be adjusted.

 I tend to add more enemies due to larger party size than the campaign is made for. 
at least every boss encounter has special conditions that trigger at ini count 20. but due to larger party sitze and aura of protection clumping, they rarely have any larger impact aside from respawning minions. 

I did exactly that. The problem: they looted exactly three full plate armors at that part (because of the three knights storing their equippment close by) and I made the mistake to allow to sell two of them. Giving them the money for the allready lower than buy adjustment price (I made it pricey and time consuming).