I don't think that there is a big possibility of Russia using tactical nukes in Ukraine. Even in frontline cities such as Bakhmut and Vuhledar, there are still thousands of civilians trapped in the basements and bomb shelters. A majority of them are ethnic Russians and in case Russia uses tactical nukes, then these people will be destroyed. And the other option for Russia will be to use tactical nukes on cities that are mostly inhabited by ethnic Ukrainians, such as Lviv and Ternopil. But this will result in retaliatory strikes by NATO.
As such there are no "tactical" nukes. Nukes are nukes no matter what they are called. Even the best tactical nuclear weapon will result in the deaths of thousands of civilians, along with the military manpower and equipment. There is no magical nuclear weapon that will only kill the soldiers. And at this point, I don't think that Putin is lunatic enough to commit such a mistake, which would result in his strongest allies deserting him.
A tactical nuclear strike against Ukraine by Russia is generally pointless from all sides. Putin doesn't really care about the Russian-speaking population of Ukraine, who will suffer from its use on the front lines in Ukraine. Now, "liberating" the settlements in the Donbass, the Russian troops are deliberately destroying them almost completely so much that it is simply impossible to restore them, and the Russian-speaking civilian population is dying there in the first place. In order to fight further under conditions of nuclear destruction, Russian troops will need to have special protection, and there are very big problems in them and with conventional equipment.
Attacking the western part of Ukraine is like attacking NATO countries, since the radioactive contamination will spread to them as well. And Putin has already been unequivocally warned that they will strike back not only at Russia, but also personally at Putin in his bunker.
In addition, militarily, a tactical nuclear strike will not solve anything militarily. Ukraine will continue to defend itself under any conditions. But it will become clear to the world that Putin's Russia will need to be destroyed and the impending threat eliminated.