Absolutely agree that if he gets away with Ukraine he will take other Baltic states.
I personally don't have the information or training to assess if he will use nuclear stuff but if he does, I don't think he would go out for full world-ending war unless he saw all of Russia disintegrating around him because they were being invaded. If, if he uses them it would be battlefield or dropping one on Kyiv out of vindictiveness. He seems to be a vindictive man. It would likely be the end of his power though because I think his own army staff would be more than a little upset and China woudl be too.
I -don't- have the training but since you asked ... I think push him back to the borders and leave it there. Accept a peace then.
But I'd also be investing massively in a cyber-security programme and putting some intelligent investment into other countries such as building roads, facilities, encouraging and financially supporting moves to cleaner energy and improving the environment. I'd be fighting a media fight to oppose the Russian influence in Asia, Africa, South America.
There are a million problems not least to do with corruption - the amount of money invested in Afghanistan for non-existent soldiers! and the desperate inequality in oil-rich Nigera! so many problems.
But I think we need to think both short term in terms of a free Ukraine, Putin left in power but boxed in, and then also in the long term.