So the larger car rental companies are in Chiang Mai? Should I book before I get there to get a cheaper rate. By 'before' I mean few months beforehand.

So about a week to see the cities including bkk going overland by train, bus, minibus, taxi.. , 3- 4 nights Chiang Mai. And finish up with phuket. 6-8 nights.

Buy flights now Chiang Mai to phuket, and phuket to bkk for returning flight home... Beware luggage charges on cheaper low cost carriers. .. Suggest you fly Thai Air or Thai smile on hkt bkk route as it goes directly to to Suanabuhmi airport, and Not Don Muong (dmk). This makes for easy connection flights and they include 20kg of luggage. I recently bought same flight to Bkk on Thai Air for 40$.. Overnighting in an airport hotel for as little as 25$ is easy if you have an early am flight and most hotels hAve regular airport transport.... Right now prices on Thai for both flights show about 77 and 40 respectively, but there is cheaper by a third.

Pretty sure Air Asia or other does cnx to hkt route. Bangkok Airways always expensive.

It's low season, rates are down.

You could check Panviman in both Chiang Mai and koh phangan, though phangan is the wrong coast. . Not sure budget. Sleeping at the elephant sanctuary??

Not sure on boutique living...

I have always advised against backtracking, going into and out of Bangkok twice is just a waste of holiday time and money. I would transit straight away to Chiang Mai on arrival - flights are cheap. Save Bangkok to the end of your trip, it puts you in situ for your longhaul back home without any last minute rushing and makes the most efficient use of your time there.

No,you certainly won't be alone,unless you want to be...Bangkok,Chiang Mai and some of the islands are always full of tourists...

We were considering the Mae Han Song loop but are you saying the best countryside is North-West of this? Also is the area north of Chiang Rai worth a visit?

No by north west I also meant the Mae Hong Song Loop. You don't have to do the whole loop you can just go as far as Soppong . Or you could go as far as Mae Sariang which has nice countryside (although view of the river from the town is completely blocked off a continuous row of guesthouses , bars , restaurants , resorts above the river bank). For Soppong recommend the village of Tham Lod 9 kms away for good scenery and walking. North of Chiang Rai is worth visit but not as forested as Mae Hong Song area.

PICTURE - Tham Lod north village

PICTURE - Soppong with direction signs to Cave Lodge and other guesthouses

PICTURE - Countryside outside Mae Sariang

Thanks Davelliot, that is very kind. Looks like the Tham Lod village is exactly the type of terrain we're looking for!

Thanks for all of your advice on this topic, really has been very helpful indeed.

We were considering the Mae Han Song loop but are you saying the best countryside is North-West of this? Also is the area north of Chiang Rai worth a visit?

No by north west I also meant the Mae Hong Song Loop. You don't have to do the whole loop you can just go as far as Soppong . Or you could go as far as Mae Sariang which has nice countryside (although view of the river from the town is completely blocked off a continuous row of guesthouses , bars , restaurants , resorts above the river bank). For Soppong recommend the village of Tham Lod 9 kms away for good scenery and walking. North of Chiang Rai is worth visit but not as forested as Mae Hong Song area.

PICTURE - Tham Lod north village

PICTURE - Soppong with direction signs to Cave Lodge and other guesthouses

PICTURE - Countryside outside Mae Sariang

"I have 2 weeks in Thailand, tell me what to do" doesn't usually get many helpful responses. Hundreds of possible recommendations. Read up on some of the popular destinations and try to make a bucket list. Maybe Bangkok, Chiang Mai and 1-2 other islands/beaches.
https://www.lonelyplanet.com/thailand

I have always preferred Chiang Mai over Bangkok though I would head out of it for a couple of days. I would leave CM and Bangkok as it is for now and adjust if one place had done its dash.