Andrew, your Mamiya RB67 camera is already a better, more versatile, and more productive camera than any Hasselblad.
Look at some RB67 features:
Bellows focussing means every lens does close-ups. The wonderful Hasselblad 150mm portrait lens doesn't focus close enough without an extension tube.
Leaf shutters for flash synch.
Best mirror damping system ever, no mirror slap, no camera shake.
Lenses from fish-eye to telephoto at moderate cost.
Rotating back means never having to turn the camera on its side.
67 format negatives naturally fit standard rectangular paper sizes. Square shooters 6x6 have to crop and waste film area to do the same. The 'blad becomes a big, heavy, expensive 645 camera for rectangular formats.
Weight doesn't matter 'cos it's on a tripod.
Purely mechanical system, no electrics, no electronics to fail. No tricky matching of components to avoid a jam. The RB67 should be repairable into the indefinite future.
And it's not a veblen brand that asks stratospheric prices for mundane accessories.
And if you really really want to shoot square you can do it with the RB67. Just get a P Adapter plus a M Adapter and a Roll Film Holder from the Mamiya Press system.