I read with interest your list of various camera's and methods. I do agree that the modified slides can be fussy and hard to manage.
However, after looking over your list, I contend that you have the closest thing to what you are looking for in your possession already. Just leave out the need for working with roll film... less emulsions anyway and mask a piece of ground glass for a 6X12 from the middle of a 4X5 sheet:
1) no managing the cut dark slides
2) consistent 6X12 exposure
3) movements, which you are already familiar with.
4) The film holder mechanism is familiar, and you can use readyloads/quickloads.
5) The waste is negligible.
Otherwise, it appears your option, if you persist in movements, is to have a 4X5 customized to a rollfilm back, making it a single purpose camera, and expensive to boot, I suspect.
If you forego the movements, then I offer one other suggestion that I have done. Shoot with a Fuji GM670 or GW670 and stitch with Pano SW, for 6X12, or shoot two images with the GW690 and scan stitch for 6X17.
Yes, I suspect my desire for one-off cameras (call that obsession) has led me down many paths where the roadblocks have been cost of the project. Hence I often succeed in jerry rigging something quite less expensive to get the same result. For me there is much pride in making cost manageable and getting the project done. When I buy the obsession driven custom model, the pride of ownership of a one-off model is less rewarding, AND I am broke to boot.