Just Want Honest Opinion from this

Should I just stop playing at around level 10 and start a new campaign?

Yes, I highly recommend that. Most published adventures end around level 12, because the game gets kind of dumb / OP beyond that. 

Also, what should I say to of my players when one of these spells is used against them and they are prevented from playing an entire combat?

The players should have ways to overcome these spells by the time they are facing enemies that have them, alternatively simply telegraph to the players that an upcoming enemy has these spells so they need to prepare for them. Most of them have pretty easy counters if one can prepare for them.

Hypnotic Pattern -> Magic Missile instantly wakes up 3 allies, it can be Dispelled, or the awake characters can break the caster's concentration.
Shield -> Saving-throw based damage/abilities completely circumvent Shield, several spells can take away a creature's reaction preventing them from using it, or worse case scenario it can be Dispelled. However, usually buffs to attack rolls + saving throw based abilities will overcome Shield.
Forcecage -> a good CHA save + teleportation gets them out, alternatively as of 2024 you can break the caster's concentration, for the bar-version polymorph, or gaseous form can allow them to escape. I believe Disintegrate also can make a hole in it
Simulacrum -> Dispel Magic and Antimagic Fields, instantly deletes a simulacrum. However as a DM Simulacrum is an awesome tool to allow the players to interact with and fight/kill "the BBEG" without actually killing the BBEG.
Magnificient Mansion -> Can be Dispelled, or one can simply set an ambush outside the door. If used in combat enemies can easily push a table in front of the door to give themselves Full Cover from any PC ducking in & out of the door. Or the enemies can walk to a different room and continue their dastardly deeds.
Demiplane -> Can be Dispelled, or enemies can open the door and go inside to fight the caster.
Leomund's Tiny Hut -> Can easily notice and Dispelled, or see the above
Etherealness -> Magnificient Mansion and similar magically secure hideouts (e.g. Temple of the Gods) prevent even ethereal creatures from entering. 
Wall of Force -> Disintegrate destroys it, Misty Step and similar abilities can teleport through it, or you can break the caster's concentration.
Banishment -> a bit tougher, but breaking concentration still works and it is only a single target.
Forbiddance -> Teleport next to the area and walk to your destination, 40,000 square feet is only an area 200 ft x 200 ft which takes a horse 2 turns to run completely across.
Nystul’s Magic Aura -> This one doesn't really have any offensive purpose? Not sure what your concern is about it, unless you've been watching "influencers" talking about niche cases that never should come up in a real campaign.
Polymorph -> reduce the temporary hp to 0 by attacking them, Dispel Magic, or break the caster's concentration
Private Sanctum -> Again this is a tiny area, just teleport next to it and walk in. It's also very useful for DMs to have in their tool box to stop the party spying on the BBEG all the time, or using Dimension Door or Teleport to skip an entire dungeon and teleport directly into the BBEG's bedroom.

Also, what about players complaining about how certain character is too strong and their character suck?

Try talking to the players, if they have any degree of maturity they will realize that playing with a suboptimal character is better than getting kicked out of the group for ruining the fun of the other players. Alternatively, powergamers are usually willing to help the unhappy player rebuild their character to be more powerful - we just did this in one of our games because one of the players was unhappy both with the power level, and the style of play their character had, so we rebuilt their sorcerer into a rogue.

It seems to me that the root cause of everything are the OP spells and items that only certain players have

Items should NEVER be given to players unevenly, and you as the DM should completely understand the balance implications of any item before allowing anyone in your party to have one. Magic item balance is even worse than CR balance in the published material. For a new DM, I'd recommend only giving out published Uncommon items from the Dungeon Master's Guide (and none of the customizable ones), until you have a better grasp of game balance and encounter design.

Since it sounds like you have some powergamers in your group, and you are a new DM, I'd suggest the following rules:

1) No multiclassing - Multiclassing can be powerful for the skilled & experienced and can lead inexperienced players to accidentally cripple their characters.

2) Use only the Player's Handbook, the Dungeon Master's Guide, and the Monster Manual -> don't let the players dig through every published book to find the niche OP items or character options that were specifically designed for one particular adventure.

3) No magic item crafting, and no magic item stores -> keep magic items limited to what you choose to give them, and make the players work for them.

4) Change the Shield spell so it can only be cast by characters who get it from their class/subclass (i.e. Sorcerers & Wizards) and only if they are not wearing any armour or using a shield. Don't allow other characters to get it through feats or magic items.

5) Once players are 7th level or higher every BBEG should have at least one Legendary Resistance.

6) End the campaign ~level 10-12.

This isnt about power gaming (valid playstyle btw) its about lazy DM'ing if you strip 1/2 the game away it makes whats left easier to run. Ive never had an issue and I allow everything - when someone uses something in a way detrimental to the session we have a chat about and i put a stop to it banning all that stuff is meh -

Ffor example I made Simulacrum a 9th level spell and made the player recast it every day in order to keep the sim - problem solved - he did it but at the cost of his 9th level slot.