It's one thing to speculate that this is what would have happened, call it one's "gut feeling" or "character read" or even "reasoned speculation", and leave it go at that.
But to be "absolutely certain" about a pure hypothetical like this (concerning a war that dumb and irrational for Putin to start, in any case) seems, well -- quite strange.
It's also unlikely, given that one of the key drivers for Putin's decision to launch the full-scale invasion was likely (not proven of course, but by any analysis it does seem highly likely) Trump's isolation stance in Afghanistan, and blatant backstabbing of the local Afghan government. This surely emboldened Putin, convincing him that a new era of disdain for interventionism had take hold on the US side -- and that he could most likely go in and have his way with Ukraine, with no significant consequences of any kind.
And Israel invades Gaza every year, under every president. It's just that in 2023, someone decided they had the propaganda power to make it seem like a new thing and that it was Biden's fault.
Did Biden "start" the Israel-Hamas war? Or even do anything to conceivably precipitate it?
Since the answer is "no" -- why does this count as a war "started under Biden"?
So long as the war in the middle east or Ukraine does not involve US soldiers on the ground, Trump can finance or equip one of the side - for the average voter in the US, there is no "war".
Maybe the the young men in the US were more concerned about the war in Russia escalating to a conflict that would involve US soldiers on the ground.
We know how Trump will behave with Putin (he will offer half of Ukraine on a plate in exchange for pinky promises.)
We can suspect that Trump will not move a finger when those promises are broken and the Baltics are invaded.
What is still a mystery is how Trump will deal with Iran - here, there is no clear policy that will please both Israël and Russia, so someone will have to give.