What gets me about pavise crossbows is they use the pavise from their back. There's not much historical support for that, not much rational support for it either. Not only would the shield have to be lighter than most pavise were, being directly on the body it would be less effective for its thickness as well. (No air gap.)

I really think pavise should be handled like shield equipment; slows them down but offers protection and accuracy advantages, but they can drop it to fight as "normal" crossbows instead.