If by R mount it is meant Canon RF, I see no problem; if one goes the way of adapting, there are plenty EF lenses to adapt to RF - both from Canon and Sigma.
It is obvious why Sigma decided - for the time being - to focus on the Sony E mount: it has the largest user base. And the L mount goes without saying: they are partners on it.
BTW #1 - so much for the spiel that the Sony E mount is too small to allow for fast lenses.
BTW #2 - interesting Sigma fp camera. Could do well with the 45 pancake lens.