The Donbas and marine offensives are going to be critical. The odds should be more in Russia's favor in the East and closer to Crimea than they were in Central and Western Ukraine -- key word being should, not necessarily the reality, though.
But in terms of Putin's future, change should to must. If Putin is to sustain his regime beyond the next year or so, it's not that he should win in the Donbas and Odesa; it's that he must win, especially in terms of holding the Donbas and Crimea.
If Putin's army suffers catastrophic losses in areas they now hold similar to the ones they had near Kyiv, that would be an unmitigated disaster that Putin may not be able to recover from. And that is why I am somewhat of a contrarian, and why I've reversed my own position since the start of the war, and now advocate that NATO become directly involved. I think NATO needs to slaughter the Russian army and absolutely annihilate any every Russian soldier and every piece of Russian equipment inside Ukraine - not in Russia of course but within Ukraine's internationally-recognized borders.
Russia cannot be allowed to have a face-saving exit and time to recoup their losses and think about how they're going to meddle in more elections and make more war. The war machine has to be crushed - because this would be a shock, a terror, that Putin and his inner circle have never before faced, and I think it would lead to a catastrophic loss of internal confidence within the military and national security agencies that would erupt into open civil conflict. And it wouldn't be long before angry, poor Russians would start taking sides, and most would side against Putin, IMO.