Right, I thought it was apparent the further Russia gets from their border the more compounded their resupplying issues get. That, plus a denser urban environment that allows ambush tactics, gives Ukraine the advantage the closer you get to the capital.
Even if Russia eventually secured the Donbas - which can happen but definitely not a given, it will be hell for Russia to hold it and will require a massive amount of resources to occupy long term. I think the rule of thumb was one soldier for every 40 or 50 citizens in an active insurgency. The Donbas region has 4 million people - so Russia could need around 100,000 troops just there to hold it while they try and annex the region (which could take YEARS).
Russia is dealing with the “simple” part of the Eastern invasion right now.