Russia is more likely to annex Belarus than Ukraine, given that it's already eight tenths of the way there and moved troops into it with far less muss and fuss. That said 100,000 troops isn't nearly enough to launch an invasion of Ukraine against even mediocre resistance and any halfwitted Russian regime would know that. If the Butcher of Grozny did launch that kind of invasion he's far more likely to impale himself on a badly led Ukrainian army than he is to win it because a Ukraine invasion is not a small task. And Russia's record of armies in cities since the end of the USSR (and in Afghanistan before its fall) is mm, middling.

Ukraine could fight the war pathetically badly and still win it. Russia has to fight it superbly or it will see Putin break his regime. Whether or not enough rational people with influence are left to point that out to him and live afterward is a different question entirely.