> Putin wants to take Ukraine largely intact to control its population and resources, not turn it into an irradiated wasteland

Nuking Kyiv and Lviv would probably force Ukraine to surrender.

> use of WMDs would invite economic sanctions from key neutral trading partners such as India

This is the real constraint. There is still the credible threat of being economically isolated by America, Europe, China and India. (Neither China nor India wants the normalisation of nuclear bombardment normalised. They both have border disputes with nuclear-armed neighbours.)