Why not go the other way...allow the monk Wis AC to apply even if holding a shield. Then everyone is in the same boat. Monks still wont hold the shield as they lose too many benefits from being uncentered. But at least then S&B and TWF get the same benefit from monk splashes and it isnt just the domain of the TWF.
The other route I like is to cap the AC bonus to the lesser of monk level or wis bonus. The only issue there is that it will nuke a lot of people's builds as they will have overinvested in Wis for their monk splash.
In any event, nerfing the Icey raiments to be a natural armor bonus or some other bonus would be good also...
I also like adding more shield ac bonus crafting rituals. Up to +4 would look good to me.
There are lots of options...