Belarus is nearing an internal revolution as it is. I'd be surprised if any Belarus troops are actually pulled into the war against Ukraine.
And if Belarus DID make a move, I am pretty convinced that Poland will find an excuse to storm the border and end it, with haste. They wouldn't be making a move against Russia proper so dickface putin wouldn't have any leverage for engaging against them.
Honestly I'm more worried about Poland right now, than I am about most other factors. They're better armed and better-trained than Russia or Belarus, and they really, really hate the Russians. They are just itching to get into it.