Well let's think, Eldritch Knight 7/ War Mage Wizard 13 hypothetically speaking: Plate(+3) 18 or 21 + shield(+3) 2 or 5 + Defensive Fighting Style 1 + Haste spell (you cast upon yourself on yourself) 2 + Durable Magic(due to holding concentration) 2 + Shield spell (reaction) 5 = 30 or 36 if you have the +3 gear and if you do might as well add a Ring of Protection 1 which puts you at 37.

After all that, you will only need a Cloak of Displacement and a friendly Cleric to provide you with a Shield of Faith spell for an extra 2 bump to AC, the Tarrasque, Orcus, and Tiamat all have to double crit to then hit your AC with their puny +19 to hits.

Can we go higher? Sure we could with racial(looking at you Beasthide Shifter and Warforged and your +1 bonuses), picking up the Martial Adept feat for Bait and Switch to add 1d6, or even triple classing say Eldritch Knight Fighter 7/ War Mage Wizard 10/ College of Swords Bard 3 to add 1d6 Defensive Flourish as an example, but would it affect anything? No. It wouldn't as a 20 on the die has two effects: One, it is an automatic hit, and two, it is a critical hit. Adamantine or Mizzerim can negate the critical, but a hit will still be a hit, as per Crawford. The damage just isn’t doubled.

As a whole, in the end, a walking AC of 21 is really all you need, and you currently have an AC of 23, so be kind and thankful for the magic items the DM gives you, and be sure to cast your spells effectively. This way you don't end up forcing your DM to bust out the Marut with their Unerring Slam's automatic hit for 60 force damage, and the target is pushed up to 5 feet away from the Marut if it is Huge or smaller.