I think the moral position is to advocate to minimize the number of lives lost and injustice done over all, not just the number of lives lost during the war. But I agree all other things being equal, a less life costly war is better.

Russia does pretty aggressive demographic reshaping to areas they've conquered. Like in Crimea, they moved much of the local population and replaced them with people more loyal to Russia. It isn't just to move people around like that, uprooting peoples lives for more favorable demographics. Plus the stress of being forced to move to somewhere unfamiliar is also quite difficult, causing some shorter lifespan. If Russia conquers more territory, they'll likely do the same there. That will lead to increased distress and death. So that needs to be weighed against the cost of war.

The bigger concern though is the longer term outlook of Russia. If Russia is able to start more wars, that will kill many many people. So we do not want an outcome of this war that will permit more wars to result. If Russia is right and Western support for Ukraine just dries up and they can take territory and people, Russia will be more inclined to try it again other places in the future. This invasion was based off of their successful invasions of Crimea and Georgia (sorry brain slip) for example.

I would like a quick end to the war, but not one that sacrifices many civilians or leads to more war in the future. So I support giving Ukraine as much aid as is reasonable to quickly win the war to minimize loss of life.

Why just war theory matters is because the public still influences policy in democratic countries. If a war is unjust, the public is more likely to be against it, which is bad for reelection chances of those involved. I agree we shouldn't take the government's justification at face value, but instead form an opinion based on our moral framework. So it's still useful to critique the government since they need to listen at least a bit.

Edit: also, the death of civilization, especially children is usually more unjust than the death of willing combatants. This effect isn't huge, but minimizing civilian casualties is better than minimizing military casualties.