I would prefer to stay in Chiang Mai over BKK, its more peaceful and low key, quality of life for the money a better value IMO. I would think access to fresh veggies/fruit/produce would be ideal too, and also the air cleaner for outdoor exercise.

My niece and husband, are in Nicaragua, arrived 3/15 and on a surf trip, they are now in lockdown, and the only guest at chic small beach resort, and have agreed to look after the place for free room and board, (6 cabinas and a pool) along with a night security guard, so thats a win win IMO. They live in Orlando FL, moved from Cape Town South Africa 3 years ago..

Nicaragua is rather deserted of tourist since the violent riots/protest and crackdowns last year, so many expats left and a lot of places shuttered, the only surf scene there is now Californians and surfer/expats in Costa Rica enjoying the desolate surf breaks/beaches....

Krabi Town
Trang Town
Prachuap Khiri Khan
Kanchanaburi
Chiang Rai
Phayao
Mae Sariang
Mae Hong Son
Nong Khai
Chiang Khan
Khon Kaen
That Phanom
Ubon

Plenty of "culture" in these places

If I was you I'd choose 5 or 6 places and do a week in each.

I think the point is retirement in Chiang Mai, and respiratory problems. When I was younger, I thought of retirement in different countries, but I get more bang for a buck right here in Florida. Also, health care can be an expensive proposition. My friend who lives in BKK has Medicare, but he has to return to the USA to use it. It's not fun getting sick in a foreign country. I wasted 7 days in a hotel room in Vientiane with a stomach virus once. It wouldn't seem bad if I had a 3 month trip, but I only had a month.

but most years it doesn't get to the stage where its the most polluted city in the world briefly.

How do you know that?
Any sources or just your own guesstimate?

There's little on historical pm2.5 data out there but pm10 back in 2013 was sky high, too - higher than this March probably:
http://siamandbeyond.com/smoke-pollution-makes-march-the-worst-month-to-visit-chiang-mai-northern-thailand/

I have seen another graphics over several years, with similar pm10 peaks every year. Sometimes the spike is a bit higher, sometimes it's broader (longer duration) but overall rather similar.

Thank you for answering my questions. No sunshine in Tioman till end of Feb. Chiang Mai is too cold for old man like me. And Thai baht gone up so much. I guess I just stay home drink more papaya milk shake. How I miss my papaya milk shake when I travel. Good day .-bonne journee

I have never paid this much for a nice guest house in Bangkok in fact I find it cheaper than Chiang Mai as there is much more choice, when are you going?

Thank you richiavo for your answer.
Actually the reason for spending the two fordt days in Bangkok is to rest from a 15 hours flight from Morocco instead of taking a second flight the same day although it is a local and short one.
This said, I wanted to take the train for the mere experience of it but somebody told me that a 12h trip without much to see since it will be dark with little to no sleep is not the best of ideas. So thinking of flying fr Bangkok to Chiang Mai even though this will shorten my stay in Chiang Mao due to airport and travel transfers so I guess Im sticking to 2 full days only for Chiang Mai. Is it fair for Bangkok?
Also, is staying in Koh Lanta as a base for my day trips a good idea or should I do some island hopping and spend 1 night at each island?

hesitant on Indonesia because I've heard it's fairly dirty
cultural locations over over-crowded/seedy cities.

Dirty? Seedy? Too each his own, I suppose. However, if these are the sorts of concerns that you are carrying with you, then be prepared for a huge dose of culture shock wherever you go. All the more because you are spending most of your time in Chiang Mai and Bangkok, all the while wanting "cultural locations". I'd agree that much of a country's essential culture is found in ways and places that you apparently want to avoid. But, again, to each his own ..

It is the rainy season in Indonesia, probably the more relevant consideration. I rather enjoy the rainy season in the tropics, but it is something to consider if you are looking for a sunny concept of a beach holiday.

Other than that, 3 weeks isn't a lot of time, and there is indeed more than enough culture in Thailand, in several guises. Is it worth spending the extra time and money for just another superficial glimpse of culture in a nominally different country? Indeed, why go to Bali for such a short period of time and then try to plan in visits to "cultural locations" on neighbouring islands?

Start the trip with a beach break if you wish, but consider researching other options/possibilities for whatever you seek that are closer together geographically in Thailand between Chiang Mai and Bangkok and visit them on an overland trip back to Bangkok. The archaeological temple sites at Sukhothai and Sri Satchanalai, for example, or Ayutthaya. The last could be visited as a day trip from Bangkok, but at least one overnight there is worth considering .. depending on how much culture you want in the form of archaeological sites.