I have, for some years now, been using Mamiya 6 RFs and they are as light, compact and handholdable as just about any 35mm SLR with a vertical grip... including lenses. Handholding is a total nonissue. In fact, the 6 is my #1 favourite handholding camera for landscapey stuff, I chose it over the 7ii because it is collapses so well. Almost no reason to put it on a tripod, you can get exceptional handheld images down to 1/8 sec.
I use an rb and a 6 for most of my MF, they are totally different tools. I just started using a 645 AFD and that is again a totally different beast- much lighter than the rb but way bulkier than the 6. The main advantage of the 645 is the much faster lenses.
For the things I normally shoot with the rb, the heft is a big plus. I mean, for macro, you want a very stable, bellows-focusing camera.