Make bucklers get their AC total added to until it reaches +6, but never exceeds it - if the wearer has parry ranks.
Why? Firstly - shields are not static things IRL. All shields intended to actually keep you from dying require mobility in your arms and a buckler was quite literally the concept made manifest of trying to use the center bit of a shield to actively catch the blows and ward them off.
Bucklers are also very niche and under-utilized. Giving them an interaction with parry allows them to be used aesthetically as well as gives you more gearing potentials for cool little items forgotten to time in the loot matrix.
Make tower shields the same exact AC as large shields (ie, inherently +2) - but when you stand still you activate a 15% concealment against ranged targets. If you have expertise, and IMP expertise, add another 5% for a total of 25% concealment against ranged while not moving and not in combat.
There is quite literally no reason at all to use large shields because the Serpent's Woe Aegis exists - but also tower shields both in PnP and similar designs IRL were actually a form of mobile cover. Don't get into the semantics of kite shields which are actually quite light and mobile things. T-Shields actually weigh like 45 lbs which is only something someone with DnD stat strength could ever hope to actually use as a shield... and they still would have to be agile with it!
TL;DR = tower shield is +2 inherent AC + the ability /while standing still/ IE being flatfoot, you can get 15% and upwards of 25% concealment from ranged attacks until you enter combat or do an action.
Given its a game and rule of cool - and to allow people to choose a style rather than out-right +1 more AC forever, make it so it's entirely aesthetic.
Leave large shield alone, its perfect and the middle child.
Oh - and probably make it so small races do not max out AC because they're small so +6 AC becomes +5. Large shields could allow small races to do the tower shield thing. Why? Small races have an inherent in engine +1 AC.
This would make everyone with a shield the same without disregarding a small person's non-shield AC.