I have the New Mamiya 6 and the Fuji GF670. I would strongly recommend either but realize that the 6 won't fit your needs since it shoots 6x6. But it is such a sweet camera! And much more portable than the 7 or 7ii because of the collapsing lens mount. Also no need for external VF. The lenses are excellent as well. And of course the GF670 is limited to one lens, but it is still a fabulous camera to work with. I also have the very old Mamiya Press Super 23, and it is really fun to shoot. But admittedly it is very big and bulky for a rangefinder. But it does have exchangeable lenses and exchangeable backs for a variety of frame sizes. You mentioned older Fuji rangefinders with exchangeable lenses. Fuji has been making top notch medium format rangefinders for a long time but the only 6x7 one that has exchangeable lenses is the very early GM670 along with the G690 and GL690. These all date back to the late 60's and early 70's. Everything after that had fixed lenses.

You asked about 645. If you do choose that route there are more options but first you have to decide if you are ok with the smaller frame.