UngeheuerLich
Legend
Most of the monk abilities (specifically, the martial arts damage dice, the ability to attack with Dex, and the free bonus action punch, don't work while you have a shield equipped, so you're capped at AC 16. I don't think that's good enough.
(Edit: Realized this wasn't what you meant, and 18 AC comes from 20 Dex. My bad.)
Reading my post it was a bit misleading though.