They seem to be hitting logistics limitations already with the existing forces. However, it seem standard practice in militaries to have prolonged campaigns with a majority of your force backwards in reserve and rotating them; e.g. when your first units get worn down (e.g. being at 90% capacity due to casualties) if you hold them back and have fresh ones take the front, that helps.
Rotation is not part of Russian strategy/approach. Bloodbath is. Common Russian expression is "woman will birth more". Also it looks like there were expectation that after initial cruise missiles strikes it will be walk in a park like in Crimea circa 2014. Mixed with delusions about state of Ukrainian military and general mood of population (big chunk of Russians sure that neo-nazi minority rule over rest of population for militia and population wants to be freed)
A main concert for Putin could be to prevent a Russian civil unrest igniting at Moscow at any cost.
This, Not Ukraine, or China, or NATO is probably the worse nightmare for him now.
Because it isn't. Russian main force in Belarus is stuck with no fuel, and supplies.
The rearguard units would likelly be even more soft, and incomplete, and they don't want to advance knowing it.