There is more to using nuclear weapons than that "We have them, let's use them."
Not anymore. Look at the slow progression from MAD during Cold War 1 - nuclear weapons are terrifying and for deterrence purposes only - to current where openly discussing the use of nuclear weapons against non-nuclear armed nations is no longer taboo. The more you normalize the possibility in discussions - seriously or not - the less taboo the next step of actually deploying them becomes. That is a slow progression as well, but think about this: The last American President to personally witness an above-ground nuclear detonation was John F. Kennedy. I don't know who the last Russian leader was to witness one, but certainly not Putin, and I doubt very many people in leadership positions on either side have ever witnessed one go off. I think this is dangerous. The damage a nuclear weapon can only be visualized in the abstract. I maintain those who actually witnessed them in the mid to late 20th century would have been much less likely to casually discuss using them, let alone actually doing so.
What would be the geopolitical repercussions if Russia used nukes in Ukraine?
More sanctions. A strongly worded condemnation from the UN. That's it. Here is how nuclear-armed nations would react to a Russian use of a nuclear weapon in Ukraine:
Pakistan: Our nuclear weapons are only for killing Indians, so we don't care.
India: Our nuclear weapons are only for killing Pakistanis, so we don't care.
Israel: Our nuclear weapons are only for killing Muslims, so we don't care.
China: Bad Putin! Bad boy! However, it doesn't affect us, so we don't care.
North Korea: Doesn't affect us, so we don't care.
(But can we have some of the scientific data from that?)
France: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine.
UK: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine.
US: Bad Putin! Bad boy! We care, but we're not going to risk a nuclear war with Russia over Ukraine.
(Fuck, that was cool, dude!)
Iran: Can we have one of those, please?
Russia risks no actual military repercussions for using nuclear weapons in Ukraine. The only POSSIBLE exception to this would be if a NATO country was impacted by direct radiation or fallout, and even then I don't think it's likely.
The overall general global nuclear threat is rising, even if there's no indication that something is imminent. Personally, my own DEFCON level hasn't dropped below 4 since Russia invaded.