I'm not sure that is the best case anymore. There are 22 individual Republics within Russia, and perhaps the best option here is internal rebellion. If even one of these Republics begin to become a problem, the potential exists that more would follow suit. Russia is overly dependent on natural resources, and if any of these start to threaten that, Putin would be forced to consider and deal with internal problems first. If this were to last for any length of time, quite frankly he doesn't have the military resources to continue in Ukraine (as much of this has already largely been squandered). Additionally, he would likely have to seriously start mobilizing actual Muscovites, which causes him potentially even bigger problems.
In the end, Ukraine's best bet may well be to fight until an internal collapse/rebellion is underway. In my view, the best possible outcome is the dissolution of Russia, with Russia proper contained east of the Urals and largely devoid of resources, and we all have to learn new countries, their spellings, and capitals. If that were in fact to all happen, Putin or no Putin, they'd be begging to make horse trades with the West, which gives us the best possible chance at a major disarmament (that part wouldn't include a living Putin).
Slow decay and rot leading to dissolution is the best possible outcome, at least to me. We'd then have lots of new problems on our hands, but hopefully ones we could resolve peacefully.