According to Ukraine, this is Russia's expected war plan:
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December/Jan is usually not a good time to launch an invasion. The downtick of units in October makes that clear. Interestingly, the last peak deployment was April, which would be a likely month Russia would choose in 2022.
In any case, Russia has to weigh the cost/benefit of severe economic sanctions, against the strategic imperative of securing this flank and port access. If they go for it, it's gonna cost them economically, for years. But in the long-run, they may have deemed it necessary.
As for the success of the actual war plan if executed... It's a bit tricky, since there's no surprise element here. Ukrainians are dug-in and are expecting an invasion. The Russian army's maneuver coordination will be tested here, they haven't engaged in multi-division maneuvers like this since WWII. But Russia is still expected to win decisively, granted everyone else stays out of the conflict.
With that said, I think there's also a possibility of a massive insurgency after the invasion. If the captured territory turns into another Chechnya, it'll be a disaster.