Sirius Glass
Subscriber
RBs and RZs have interchangeable backs. So, for that matter, does my M645 Pro though the earlier 645 versions didn't. Much as I enjoy shooting with my TLRs if I could keep only one camera across all formats that would be it, because it does it all for me. It's almost as good as 35mm at "35mm style stuff" just bigger and heavier, and that not even much or at all compared to the largest 35mm SLRs but there isn't the huge choice of lenses. Most of the time I end up cropping my 6x6 negatives to 6x4.5ish anyway (not always, but probably 60-70% are cropped, some I do print square.) My RZ and 4x5 I'd vastly prefer when working off a tripod for stationary subjects but again that's not everything. The 645 can do it all at least passably well for me. But more often I end up picking one of the other cameras for more specific reasons.
Hence my recommendation to move to Hasselblad to have the selection of lenses and end the frustration.