They wouldn’t do anything. There is zero evidence or even probable cause to assume putin would advance beyond ukraine. It’s been known for decades that ukraine was Russia’s red line. We crossed it over and over. We antagonized and caused this entire war. Putin isn’t some mad man who did this on a whim, and he isn’t out for world domination.
He just doesn’t want NATO on his doorstep. It’s understandable.
You know Putin added 832 miles of NATO border right next to Russia's second most important city (and his home city) with his invasion of Ukraine, right? You know he has moved his troops from this new sensitive NATO border to Ukraine because he doesn't view it as a actionable threat because NATO is a defensive alliance, right?
You know a second, technically advanced country which proudly held it's neutrality also joined NATO, turning the Baltic Sea into Lake NATO due to his invasion of Ukraine, right?
You know he views the breakup of the USSR as the biggest geopolitical catastrophe of the 20th century, right? Furthermore, he doesn't view former USSR countries as independent states or the citizens as sovereign, right?
You are correct that he isn't a mad man. He meticulously planned weakening operations in Eastern Europe, economic blackmail with Western Europe, made savvy domestic economic reforms to brace for a possible sanctioned economy, and conducted military/paramilitary operations where he thought he could get away with it.
Prior to the invasion of Ukraine, one could easily make the argument he was the most savvy geopolitical leader in the world. He did more with less than any other leader.
His only mistake was surrounding himself in a very small information bubble which persuaded him that Ukraine would be a cakewalk through a mixture of overestimating Russia's military capabilities, underestimating Ukrainian military resistance, overestimating FSB/SVB's Ukrainian corruption campaign, and the Ukrainian people's will to not be governed by Russia. The plan was for a quick strike and fall of Ukraine which would have rendered European and American help a moot possibility. When that didn't happen, European and American support, along with Ukrainian will power and ingenuity, resulted in a string of embarrassing losses.
Now, Putin is stuck without an off ramp to save face. His only hope is to continue to solidify power domestically and hope to outlast American and European support. While he might succeed on the former, he's increasingly looking like he's losing on the latter.
Tl;Dr, you have a tenuous grasp of the geopolitical reality of the Russian invasion of Ukraine.