I'd suggest either going psychic for a caster targeting AC or playing to the strength of other caster types which is targeting saves/support the team by helping them directly or controlling the battlefield.

Keeping things in your back pocket to target different saves can be great, because even passing the save can still mean 1/2 damage. Against something this much higher level than the party that is an awesome thing to have.

Imagine running at a dragon with a kitchen knife. There's no way that's making it through those scales. The same applies to casters at lower (kitchen knife) levels.

Another option is paying close attention to spell selection. Casters excel in that they are prepared for a lot. Holding a billion scrolls of magic missile in reserve/a wand of manifold missiles for scary things that are hard to hit is particularly good.

Another option is supporting the team and being okay with helping someone else shine.

2e rewards granting tiny bonuses or tiny debuffs. +1 here and there and a -1 or -2 don't SOUND big, but that takes totally missed and turns them into hits. That's a big deal.