I don't think Russia is going to do anything rash. Ukraine has been hitting targets using Western weapons "inside Russia" (what Russia considers Russia), ever since they got the longer range stuff in 2022, I think even earlier.
Any rhetoric from Putin that makes a distinction between say, Donetsk/Luhansk/Crimea and Belgorod, is a tacit admission that Putin does not think those territories are "really" a part of Russia in the same sense Belgorod is.
So we can easily throw that back at him, if he argues that.
Also McFaul suspects that Putin's strategy is based around Trump potentially winning the US presidency in November and bailing him out. Him dropping a nuke on Ukraine would be highly improbable before then, Russia would become even more of a pariah than they are now and you'd probably see neutral states like India, probably even his buddies like China start to distance themselves from him.