Why not just use the current Canon mount with an adapter?
Has anybody been using current Sigma lenses with the R? I have a macro, but I just realized I haven't tried it yet!
I mostly quit going 3rd party due to incompatibility problems that arose as new Canon bodies became available. I'm mostly going all Canon glass now.
Contrary, my Canon lenses always seem to work perfect from body to body, even when adapting EF to EF-M or to EOS R. No need for docks to screw around with. Just mount, and shoot.