There is another issue hardly discussed. I have friends in Ukraine. They absolute hate Russia. Its the main reason why Russia will never hold Ukraine again. Its one of the biggest plunders of Putin. That rift is permanent.
I think it has been discussed a little bit, and I would agree. This was a big loss for Russia, yet another reason why going in was a bad idea. That having been said, the split is real even within Ukraine. There is still a pro-Russian minority. But it's smaller than it was, more isolated, and with no chance of ever taking center stage politically, at least within the forseeable future. On the flip side, the same problem exists for Ukraine with regards to the Donbas. There are quite a few people there who also hate Ukraine now.
Lastly, on the question of "holding Ukraine", I don't think anyone here thinks Russia will conquer all or even most of Ukraine. In the areas Russia will conquer a demographic shift is already in progress, and it's something I've talked about in other posts, including relatively recently. You'll notice, despite what you write, that there isn't much of a guerilla resistance campaign in occupied areas, even ones that were clearly not pro-Russian in any meaningful way. Places like Energodar, Melitopol', and other parts of Zaporozhye and Kherson region. I think Russia will be able to hold what they conquer, assuming they can get a political settlement to end the war with them in possession of the territory. There will likely be some sort of right to return discussion after the war, and Russia will probably try to impose a state of affairs where people who don't want to live under Russia will be discouraged from coming back.