If your budget is decent, the Canon R5 is really not much larger to begin with when compared with the Sigma fp, does decent 12-bit raw internally, and has a decent flippy screen. By the time the fp is rigged to do 12-bit raw (and you add a small grip so you can actually hold the thing!), it’s already about the same size as the R5 (and still has a fixed screen). It’s one of the reasons I sold my fp and fp-l and kept my R5. Sometimes I thought about shoving the fp into a small space without a screen to get something, but then I remembered there’s no built-in wifi and by the time you add even the smallest cineeye/hollyland transmitter to it, it’s… about the same size as the R5. 😛
If the new BMMCC 4K can do monitoring over wifi to a phone and can do 12-bit raw to a really small external SSD (like a Sabrent Nano or something), it could work well for that kind of thing. If not, by the time you add an external screen (even the VL35) and small SSD, you have a kit about the size of an R5 (with worse ergonomics).
(With that said, your computer’s likely to be able to edit raw from the BMMCC a lot more smoothly than from the R5 - my M2 Max is just barely tolerable with R5 stuff in Resolve)
Also, I’d add in (though I haven’t used) a hacked EOS M as an option for a teeny tiny camera that can record raw internally. From what I remember, it can do some sort of 2.5k, though that might only be in a 2.35 mode or similar.