I also want to know if I can fly from BKK to Chiang Mai for a normal visit without having to quarantine. When the rules were relaxed at the beginning of June, the news reports I read just mentioned that 'Thai people' from BKK and other provinces could now travel to Chiang Mai. There was no clarification regarding expats who live in BKK or other provinces and haven't been out of the country recently, but the implications were that foreign expats were still being required to quarantine.
Unforunately I don't know of any reliable way of getting an answer to this question. I've checked all the domestic airline websites but no guidance there.