The amount of champions that are frustratingly unfun to play against has quadrupled in the last year

Laning against assassins is still atrocious. They absolutely failed in their goal of giving them more counterplay in the rework.
The bot lane meta is just garbage. Who abuses lethality more? That's all it is. There's still tanks one shotting squishies. Damage creep is still the biggest problem in this game.

I just really find myself feeling less 'I took this loss because we messed up A and B' and a lot more of 'We just got cheesed by A and B'

These are the champs that stand out to me the most when I think "Unfun to play against".

in a way that causes irritation or frustration?

I think you need to work on your current state of mind to solve your problems instead of blaming the last year

You can do this by being more specific.

I'm pretty sure the problem is Yas and Camille

If more champs are frustratingly unfun to play against the in logic the diversity has indeed gone up.

More or less I agree, riot's design teams seem to go by two different philosophies; fun to play as, or fun to play against. Without a middle ground.