Well for a start Moscow isn't the entirety of Russia, considering the effort they've been willing to put in for Ukrainian territory they probably aren't going to risk them or even "proper" Russian territory like Belgorod or Kirst being dusted with fallout. To say nothing of the fact the front line are currently well within all four of the newly annexed territories borders, the capital city of one is still only a few miles away from the 2014 lines and another was never reached at all by the Russian's initial push.
So tactical nukes used in anything less than strategic manner would be of little use and at that point the entire stated objective of the war is lost because the land and people they're fighting for will be fucked.
Building on that, using nukes to the extent it would have a legitimate effect on the outcome of the war (i.e. strategically, regardless of the yields) or even potentially much less would likely result in a conventional military response from the west that would thoroughly destroy Russia's conventional warmaking abilities and leave them in a position to choose continuing a fight they cant win or further escalating with nukes against nations that could hit back in kind.
Aside from the potential severity of a western response, any use of nukes at all in a war of conquest against a non-nuclear power would immediately put them on the shit list every other nation that even slightly matters, including all their so called allies and in particular China and Iran. Japan, Taiwan, Saudi Arabia and other gulf Arab nations are at most a few months away from either producing warheads themselves or buying them from allies, the non-proliferation treaty would die the moment Russia uses nukes under these circumstances.