it's very easy to post a simple and effective balancing on back shield
something like a modifiler making you 5% slower on all action while holding it in hand, x2 if it's on the back, x3 if both, multiplier based on shield size
this way you don't hurt the orthodox play style, the varangian guard style could be done without significant advantage and turtle lover can be a turtle
but this is taleworlds we don't do this here, deductive changes are the only choice. If it doesn't suit the vision we remove it.