Out of the cities you mentioned I've only been to Chiang Mai, and I think it can be a great base for trips within North Thailand, but I wouldn't limit myself to just day trips. I did a similar sabbatical with my partner, and we spent about a month in Malaysia (only the peninsula), a month in Myanmar, a month in Thailand (only the northern region), and a month in Taiwan. But different to your idea in that we were a bit more nomadic in each country - instead of doing day trips out of a home base, we'd just up and go to the next city/town/village we were interested in, spend a few days or a week there, off to the next one, and basically kind of do a loop back to where we started. But if you don't feel the desire to visit places that are hours away from Chiang Mai, then just staying there and doing day trips can be perfectly sufficient.