The moment you use them, you lose their value as a deterrance anymore.
If they used nukes in Ukraine, the strategic deterrence against other nations with nukes would still be in place.
They didn't use them against Ukraine because of many other reasons, e.g. because they would have taken Kiev in days, it didn't make sense to make land that you want to conquer radioactive, and the political fallout would be too big.
I think now they won't do it because China is too important to them and China made it clear that using tactical nukes is out of the question.
> If they used nukes in Ukraine, the strategic deterrence against other nations with nukes would still be in place.
No it wouldn't be a deterrance anymore again other nations, it will be an actual threat. If they actually do show they are willing to use them in any capacity, esp over Ukraine which is agression they started, other countries cannot just ignore it or bow down to Russia. It also not a deterrance to nuclear powers in the first place and in the second, it becomes an existential threat again.
As for why they didn't use it in Ukraine is primary because they have never done so. The political fallout will be suicide, there is no other option.
I don't see how China fits in with the tactical nukes thing. Your position of "Russia could have nuked, they just didn't want nuclear fallout, political fallout and China being mean" instead of "they didn't nuke because it will be unprecedented and consequeunces will be dire bodering on absolute destruction" seems weak to me.
If they used nukes in Ukraine, the strategic deterrence against other nations with nukes would still be in place.
They didn't use them against Ukraine because of many other reasons, e.g. because they would have taken Kiev in days, it didn't make sense to make land that you want to conquer radioactive, and the political fallout would be too big.
I think now they won't do it because China is too important to them and China made it clear that using tactical nukes is out of the question.