I have a friend with a Canon full frame camera whose mirror/shutter is dying. Cost too much to service for an old camera, but a Canon full frame mirrorless being too expensive, for them.

Is the only Canon full frame mirrorless the R5?

I always suggest the Sony full frame cameras, is there a good EF mount adapter for a professional photographer? A lot of adapters I've seen between brands and mounts end up slowing down the lenses or have some other compromise.

And without adapters, people seem to be vendor-locked and in a toxic marriage with a camera manufacturer that pretty much requires becoming a fanboy or whatever the non-gendered derogatory term for fanboy is.

Solutions aside from selling everything?