First, what size do you consider to be acceptable? You have a 6x6, are you willing to go smaller, into 645? If not, then you have to stick with 6x6 or larger.
In 645 there are several brands that offer interchangable backs. Pentax, Fujica, and Bronica have good lenses and interchangable backs. I have never used any of them, but many in these forums have and can probably comment on their merits.
In 6x6, Hasselblad and their copy, the Kiev, offer the same. Of course, many will argue the Kiev isn't reliable. I have never used them so I don't know.
In RF, there are some 6x7 that offer that you want. Plaubel and Mamiya come to mind. I have never used the Plaubel. I have used the Mamiya Press Super 23. I like it very much. I also think it is reasonable.
You need to decide based on either reading books, old magazines, or member input here, which system you think you might like best. You can then check ebay or keh or other places, to see what the pricing is and if it is in your price range.
Good luck in your quest. With luck, others here will give you other choices as well.