I don't think muscle memory is that different between 1-2-3 and 1-1-1. As such I don't see why it wouldn't be able to work with DRG.
MNK would indeed require the most work to adept, because it's the most mechanically ingrained with the current system. On the flip side, MNK has been a mess for many years now and hasn't seen it's gameplay evolve since ARR, so maybe taking some more liberties here and re-imagining the whole job might be an option too.

Either way, I'm only suggesting consolidation isn't bad when used to evolve the game. Because most people opposing it in here seem to think consolidation will result in simplification, which isn't necessarily true. Yet cry out every expansion when half their skill bar gets deleted, which is a direct result of NOT consolidating.