Syria is a bad example, it has been status quo for years now and war de facto has ended with Assad's victory - Turkish supported rebels are holed in the small area in north around Idlib and fanatics are hiding "under" them in a strange ceasefire. Rest belongs to the Assad or Kurds, who are not being touched only because the Turkey holds that enclave and thus is much bigger problem.
Bombings? Not that simple, you need to know what you are doing, solo missions are likely bound to fail. Assasinations? No one cares about couple dead officers or soldiers. Russia is less sensitive to losses.
Russians fucked partians in Baltics after the WW2 when there was no technological dominance and massive parts of land were basically uninhabitted. It's actually easyish - destroy the support base. Want a recent example from modern age? Tamil Tigers, very very dead/imprisoned/beaten.
Oh, and Checnya's rebels/terrorists also failed.
See, West have things like RoE's, they in general care about civilians even to the detriment of military operations (no, seriously, aside from memes and stereotypes it is like that), etc. Russians are not barbarians, but they will be much harsher than American troops in Iraq, for example. Plus you have to understand that like half of Ukraine won't even really care, literally, as long as they are left alone. And since that half is quite a bit Russian...