>>3457717[Cont.]
Lastly, the shutter. Leaf shutters save you a whole lot of money and trouble. A shutter and syncing it is really complicated, you'll have to outsource it. A big focal plane shutter for a MF camera even more so. You mentioned that the RB is something you might want to get if you had the money. Bear in mind that a brand new, custom shutter ordered in small quantities could cost you per unit as much as the whole RB camera.
Potentially the same for a pentaprism.
The most vable solution is to use mechanical leaf shuttered lenses and a simple optical viewfinder, maybe with engraved lines for a couple popular focal lengths. Focusing done by external RF mounted on the hot shoe.
You said that you're aware of the current designs for 3D printed cameras, but disappointed that they're too primitive and lack a viewfinder.
You can start your project by designing a viewfinder first, either without or with framelines. I mean that's the feature that you want mostly. If that's feasible, you can start thinking about the mounts and the rest of the camera.