They're literally still not fully mobilized. If Russia mobilized instead of what they're doing it would have loads of legal financial and political consequences. This whole war has been about asserting their national security without blowback. Hence the North Koreans coming to the party. That certainly won't strain from the DPRK.

Far from "running out of bodies", Russia has not even begun conscription. They offer incentives to volunteer, they have reserves, they have mercenary corps. All ways to dodge full war mode.