If one thing is blindingly obvious lately, it is that Russia doesn't care about their own troops, only about winning. Peter Zeihan reminds us that historically Russia needs to lose about a half million men before they consider giving up on a war.
I do not believe that is their primary objective. Should any resources survive their onslaught they are just bonus material. They have already destroyed most of the factories train tracks and a good deal of the power generation. Russia have no ability currently to replace these things or to rebuild Ukraine which would be necessary in order to make use of the lands resources.
What would they target?
The remaining power, hospitals, the remaining agricultural factories to starve everyone. That alone would drive out most of the remaining people. I believe the primary objective is to secure Russia. That requires conquering all neighboring former Soviet countries and Ukraine is just in the way and has a nice bonus water port. Should they succeed in taking Ukraine this spring then I would expect Poland and/or Finland to be attacked at some point later on.
I don't think that's the objective. The objective imo is to take down a government and country that is challenging the Russian people see them as kin and yet their getting more democratic, removing (some) corruption and moving closer to economic partnership with the EU. Putin has to destroy a brighter future for the people of Ukraine to prevent his own populace seeing that future as a possibility for themselves.
There is a lot of truth in that as well. I'm sure its a major driving force for him and others around him. They are a vestigial legacy that will cease to exist in any meaningful manor once most of the dictatorships and faux democracies fall.
At this point, I'd say it seems to be an open question whether or how much it matters to him how much of that land is actually habitable.
It also seems like a) he views Ukraine as a somewhat rebellious province—daring to defy him, its rightful ruler—and b) he views Ukrainians as Russians, and furthermore believes that any Ukrainian who does not subscribe to this view is either traitorous or mentally ill...so I could see some fairly strong arguments for him deciding that they "deserve" to be nuked.
Honestly, in my view, the strongest argument for the position that Putin won't use nukes is simply that he hasn't done so already, despite many fairly significant setbacks in his attempted annexation of Ukraine.
Using a nuke against Ukrainian military forces would be a waste since they are now dispersed and dug in. Many of those forces are close to Russian forces so a nuclear strike would endanger their own troops.