I think by far the most likely option is at least one other coup or internal secession, which causes the Russians to never technically leave but to de-facto leave as they shift their attention and resources to more existential issues.

Failing that, it’s harder to say. If Russia continues to lose ground but doesn’t give up, one could imagine a scenario where the Ukrainian advance is stopped by the Russian border. There’s not much historical precedent for an all-out industrial war where a pre-existing border is held sacred. I imagine it would be hard to tell a unit to drive into Ukraine where they’ll definitely be hit by an F-16 strike, so it would probably turn into a game of lobbing rockets and drones from well within Russia’s territory, while the Ukrainians figure out how deep they’re willing to strike back.

The other options are Russia starts taking ground again, which seems unlikely, or Russia agrees to white peace including giving back Crimea, which might be politically impossible as it’s seen as an integral part of Russia.