2.6! That's interesting. Andrew Reid, in his article on the M50 as a Digital Bolex says the crop is 2.34x while Dave Altizer said it was around about 2.45x.
Yes, the use of the 4k is limited mainly to interviews or slow pans.
I wonder if someone, like the engineer in South Korea who made the EF-M EF speedbooster, would like to also create an EF-R to EF one with a 0.71x reducer. That would reduce the 1.8x crop to something more usable (albeit you would have no access to the EF-R lenses).