Well, i understand how you feel about your RZ and Mamiya 645 bodies, i really love to shoot with my RZ ProII a lot, but the weight and the battery killing the fun, so i bought Mamiya 7II but it doesn't work, i know the battery issue is still there, but the weight is way way different like day and night, i got used to heavy gear, but RZ is way too much heavy than normal, so Mamiya 7 fits my hands perfectly.
I was looking at Fuji GF670 as alternatives, this camera i can switch between 6x7 and 6x6, and it folds nicely from what i hear, and it is almost same size of Mamiya 7 or lighter, but it is rangefinder and not SLR body at the end and i don't want to spend another money on another body and not to use it much enough, so after long search and read i found that Mamiya 7II will be the one, just hope i can fix it one day and then i will keep shooting with it for long time.
You could always rent a Mamiya 7 for a day to see how you get on. On the other hand, they do keep their value, so you could also buy one.
I recently fell for a Rolleiflex TLR 2.8, and was astonished by the fact that just that single camera seemed sufficient. It produces great results, and the format at 6x6 is great. But I still have a 645 system as well (Pentax though).
Did consider the Mamiya 6 or 7 but its reportedly harsh bokeh has made me doubt. That may be something to consider. Sharpness of the 80mm lens is, they say, unrivalled.
Still, I really love carrying just my Rolleiflex.