Instability along the Russian border works against Russian interests. Stable and predictable is how they maintain the integrity of one of the most geographically exposed borders in the world.
Fomenting ethnic and cultural splits in the border lands of foreign countries, by contrast, was historically a mainstay of British empire-building.
On top of this, Russia gets a free chip in any other diplomatic negotiations its involved in. They offer to remove troops and de-escalate the situation in return for some gain elsewhere. After their gain is secured, they can begin building up troops again and do the play all over again. The US and its allies will of course complain, but they actually benefit from this too, as it justifies their involvement in the situation and they score a political victory at home every time they get Russia to back down. It is remarkable how stabilizing regularly occurring crises can be.
NATO/Zelensky are of course reluctant to go ahead because an application to join would reinvigorate the civil war. They know the cost of giving the middle finger to Ukraine's ethnic Russians and would prefer to wear them down gradually while blaming the whole shitstorm on Russia.
Russia (and ethnic Russians within Ukraine's borders) would prefer Zelensky was kicked out and replaced with another Yanukovych who would kickstart the process of deintegrating from the European/American sphere of influence and reintegrating into the Russian sphere of influence.
Ukraine could do a better job of playing the two great powers off against one another a la Tito/Yugoslavia, but instead it swings violently between being ruled by a bunch of corrupt Russian-aligned oligarchs to being ruled by a bunch of equally corrupt European/American-aligned oligarchs.
Of course the whole situation is complicated by the fact Ukraine's borders are a product of Soviet planners who never thought it would wind up as an independent nation, but the ethnic Russian population is small and generally not prosperous. Ironically, Ukraine would probably be better off letting the separatist regions secede, but to do so would be publicly perceived as a win for Russia and thus is not permitted.
I stayed in Odessa a couple of years after the Maidan with an elderly (ethnic Russian) couple who ranted at me for five minutes straight about "the idiots in Kyiv". They all but endorsed the separatists in Donetsk. They supported the protesters initially "when it was about corruption and living standards", but never desired closer integration with the EU and desperately wished that the value of their savings hadn't plunged 80% as a result of the "coup"/"spontaneous uprising of freedom lovers"
I suppose it's always possible that this elderly couple were shadowy agents of Putin sent there to destabilize the country with a "fairly gross" misunderstanding of their own life. No doubt say, the Washington Post would see the bogeyman of Putin hovering there, as it usually does.
Nonetheless, I believe that their fury was real and shared by quite a large proportion of the rest of Ukraine (albeit more in, say, the east and less in, say, Lviv).
Not to mention that being ethnic anything does not give you the right to start a war. I suggest you imagine a situation where ethnic non-Russians, in Russia, decide to break off a piece. Does Russia have the right to ruthlessly end that? Because it wouldn't even be a question.
Phillip Karber, an American military scholar, has been to the front many times to study Russian military equipment and tactics there:
https://mwi.usma.edu/video-dr-phillip-karber-ukraine-russian...
Suppose then that Alaska, now a nominally independent state, was on the verge of voting to rejoin Russia.
I imagine the Moscow liberals would think much like you do about the "American" uprising that would inevitably follow. It wouldn't even be visible to them - they would only "see" a war of aggression being waged against a foreign state, directed entitely from DC.
Similar to how Ukraine broke away from the USSR.
So if the US dissolves into, say, Alaska and Hawaii and Puerto Rico and the "other 48", and then Alaska votes to rejoin Russia, then the 48 would be unhappy, sure. But they wouldn't have any say, because in this scenario Alaska wouldn't be part of the US any longer. In the same way, Russia has no (legitimate) say, because Ukraine isn't part of the USSR any longer.
It's kind of like how the UK didn't have any say over the US after that 1776 thing, and when they tried to act like they did, it turned into the War of 1812.
In Recognition of the soon the posted argument of the: https://en.wikipedia.org/wiki/Kievan_Rus%27
Russia failed to attract its potential citizens, to offer a cooler, more vital society other people wanted to join