Like I said with France and the 13 colonies – no country is actually saying that or has ever said that. France didn't go "yeah, we love what you're trying to do 13 colonies and support your beliefs wholeheartedly", they went "oh cool, this will help us regain New France one day and really piss off our archrivals."
Saying "oh cool, this will help us regain New France one day and really piss off our archrivals" is still supporting them. That's my point, seperatists often rely on geopolitical rivals supporting them for ulterior motives. You can't really cleanly separate bottom-up political activism from opportunistic rivals with ulterior motives, because in practice the former will generally rely on the latter. Generally when you're fighting a civil war, you don't have the luxury of turning up your nose at offers of assistance for the sake of purity. So if your position is supporting seperatists movements except when they recieve foreign backing, you're not going to find yourself supporting many seperatists movements in practice, at least in cases where they have to fight.
Ok so what happens if a government says, "No you can't secede and I don't care how many of you want to?" Nations aren't generally keen on giving up territory, especially in cases where the relationship is exploitative. Renouncing force means renouncing the threat of force, which can often leave very little leverage for a seperatist movement to work with.
Personally though, I'm inclined to agree somewhat with your point that seperatism isn't always worth the conflict, and for that reason I wouldn't necessarily agree with the stance of being predisposed to support seperatist movements. Imo, it's better to take a pragmatic view, evaluating the specific conditions on a case by case basis.
I would argue that if Russia withdrew and the seperatist movements surrendered, there would still be a conflict between the Russian speaking population and the Ukrainian government. I suppose it'd be possible for Russia to offer citizenship and relocation assistance to everyone, but it would mean displacing a lot of people and I'm not sure it's realistic. Do you have examples of historical precedent in a comperable situation?
I don't think it's unreasonable that the DNR and LPR would want to join Russia for legitimate security reasons at this point. If you want to label them as Russian proxies and Ukraine as a US proxy, I don't mind, but I think the reality is that while both are influenced by foreign governments, they also both represent some degree of genuine support.
I don't think you can extrapolate like that from a single data point under pretty different conditions.