This is how Russia loses the war. Invasions are actually the easy part (except when your armed forces are poorly trained and your generals are dummies like the Russians). The post-invasion occupation is what has sunk many an empire. There are only two examples of reasonably successful occupations. The Nazis were able to hold Vichy France by placing only a few soldiers in each town/village. If the local resistance killed one of the soldiers, a battalion would descend on the town and execute a dozen people in the town square. After that, the locals began ratting out the resistance. in post-war Japan, the US occupation was very successful because the US took over every aspect of civil society. Anyone who wanted a license to practice a trade or a permit to do anything, you had to get it from the US armed forces. Whenever locals started getting restless, US armed forces will pull people's business permits, licenses, etc. until everyone got back into line. This took a lot of manpower, but the US had plenty of soldiers available because VE and VJ were pretty close together.
Trying to occupy while still fighting a very hot war is all but impossible absent the most brutal Nazi-esque tactics. Russia does have it easy in some parts of Donbas where there is a significant number of separatists who can keep people in line. But once Russia tries to take solidly pro-Ukraine territory, it will face significant resistance that may even escalate into guerilla warfare. The main way Russia has been able to make gains has been through focusing its forces and making small incremental gains through massive bombing campaigns. But there are only so many Ukrainians that can be deported to Russia and only so long before a resistance organizes and draws personnel back from the front. That is when Russia will lose the war.