I doubt Russia will resort to nukes or chemical weapons as they’ll not win the war for Russia, would certainly lose all support Russia has from any allies and would probably result in Nato countries if not Nato itself actively getting involved
For me it depends on how much control Putin still has (or will have as they suffer more defeats due to UA counter-attacks). I can’t see Putin surviving losing the war so he doesn’t have a whole lot personally to lose by escalating – it’s just whether those around him let him.
Chemical/biological weapons seem much more likely to be used than nuclear, they are far more deniable and part of Putin’s playbook is implausible denials, he doesn’t care how absurd the denial is as long as it creates enough doubt to allow China and others to continue their support and to deter NATO from taking action.
If he used tactical nukes then it’s highly likely NATO would strike Russian military targets inside Ukraine and possibly declare a no fly zone, but they would probably hesitate if some previously unknown nerve agent was used and Russia denied any involvement, blaming it on a NATO false flag operation.