Chiang Mai is actually a pretty nice choice for New Year if you're not into crazy parties. Weather is cool in the mornings/evenings but warm during the day (a few degrees cooler compared with Bangkok).

I was there in New Year time. It is busy, yes, but not too much like Bangkok. The temples are very beautiful and the food is very good. If you like to walk, go to cafés and markets, I think you will enjoy it.

I can confirm the busy aspect (I seem to remember being stuck in traffic a few times in Chiang Mai during New Year and my driver informing me that it was typical of that time of year). But the cooler weather is an absolute delight.

The air quality now is ok. It is not perfect but it is much better than burning season time. January is normally one of the good months. March and April you should avoid.

If you book in last minute, the hotels can be more expensive, but still not so bad compared to Europe. You can try to stay inside the old city if you like culture and to walk.

Thank you all very much. This help me a lot!