>>26890861I'll admit that I know jack all about Synchro in the context of the anime, but based on what I've read about it here and a couple other places, they could probably get away with it being like Full Synchro was in the Battle Network games. As in, not an actual physical transformation, but a "state of mind" sort of power up.
It'd honestly be dumb to have yet another set of transformation models (which may or may not even look like your character), especially when you could just design more Megas.