Purely strategically, the best bet is for NATO to fund and lend/lease materiel to Ukraine sufficient to make the Russian effort protracted and expensive. Maybe Ukraine can solidify some territorial gains in Russian oblasts enough to have bargaining chips. From there it is just a game of finding acceptable off-ramps. Maybe a treaty gets signed that trades territory back to something like the “original” borders circa the beginning to the current conflict.
Russia clearly doesn’t care about personnel losses (and historically never has). But maybe if it drags on, the conflict will become economically and politically costly enough that Putin is looking for offramps.