I support separatism, but I also oppose war in most of its forms
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?
Within the context of Ukraine, the DNR and LPR didn't have the relationship with Russia that, going back to the French and American Revolution example, the American colonists had with the French. American separatists didn't become subordinate to French military leadership or to French foreign policy goals. The newly-independent Americans didn't then ask to join the French Empire.
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.
As an aside, France's support for the Americans failed them in their ambitions and led to the collapse of the Ancien Regime, which if we're to take it as indicative of the outcome and legacy of foreign-backed separatist conflicts, means that this isn't gonna be good for Russia long term.
I don't think you can extrapolate like that from a single data point under pretty different conditions.