Gaining control over a foreign country territory is not a win until there is peace and other countries recognize the annexation.
The age of territorial conquest is long time gone.
Powerful countries don't need conquests anymore. They just need to subdue other countries.
If Russia has occupied 20% it doesn't mean that it can use it in anyway.
Donbas has been in a state of war for 8 years. A province which is in state of war cannot thrive.
So besides spending their resources on those 20% of land, Russia can only steal resources like grains, steel and minerals.
The best thing for Russia to do is to find a convenient exit from the war.