Mostly, MF lenses tend to fit only the camera system they were made for. There are adapters for a few cases. My 'best' (in a technical sense) MF camera is a Mamiya 645 Pro; an SLR. The lenses for it fit Mamiya's older range of 645 cameras, but not the 6x7 RB or RZ cameras or their RF press cameras. I think Bronica systems are similar; each camera has its own range of lenses. Even with an adapter, the image from the lenses for a 4.5x6 camera are unlikely to cover the 6x9 frame.
If you are only going to get one camera, you need to look at the pictures you take, or maybe the pictures you like, and decide what aspect ratio you tend to see satisfying pictures in most easily. You could even do the art-lesson exercise of cutting out some cardboard frames; one square, one 3x4 (i.e. 4.5x6) and one 2x3 (or 6x9), and try composing scenes through them. Choose which shape you like. Of course, if you choose a big format camera, you may be able to adapt it for smaller pictures; you can get a 4.5x6 film holder for some square cameras. Some 6x9 cameras (I'm thinking about old folders) have masks to allow 4.5x6 as 'half frame'; your (fixed) lens is then quite long for the format - might suit your portraits.
Since you are using 35mm RF, think about whether you want a RF for your medium format camera. Myself, I wouldn't be without ground-glass focusing.
If you're up for something quite slow (and maybe clunky) to use, consider a press camera. I have a Graflex Century Graphic which takes film holders for square format, 6x7 and 6x9. Lenses, which have to be with a shutter, are whatever I can mount in a lens-board for it. Getting lens-boards was tricky, and I had to do some drilling to mount my lenses. The camera has a rangefinder, but which I can only couple for one lens at any time. So I had to do a little DIY making focus scales for the lenses which aren't coupled. The camera also has ground-glass focusing, and I'd use that every time for landscapes.