@Linz ***********
I can't give you an exact answer on your queries, but I can share my story for additional courage.
I just finished moving with my 7yo English Bulldog from Poland to Thailand. We took the land route from Poland through Berlin to Amsterdam, roughly 14h in total. One day rest, then he flew in cabin with me from Amsterdam, through Singapore to Phuket with SIA, as ESA (sadly not an option anymore since April - but he was allowed to travel without carrier and was a star for the cabin crew). Train trip was tough, he was getting too bored, but plane flight went smoothly, he mostly slept throughout it without any accidents for 17h.
Definitely tire him out to maximum before flight for whole week, mild sedatives are OK also leading up to flight (i.e. CBD oil). IMO pet carrier is the more humane way - some dogs like cramped spaces as it gives them comfort in stress. Cargo hold is unpredictable, and it's a long journey, I would not put my dog in there ever.
If your budget allows it-consider going business class (I splurged out, as my meathead takes a lot of space, zero regrets despite big cost). The amount of space there (A350, especially if you score bulkhead seats) and privacy pretty much means you would be fine letting him out for brief period if he is unhappy a lot.
Thailand wise - it's not an easy country for foreign dogs. Heat and humidity is off the charts. Ticks did not stick to mine in a week, but Bravecto/Nexgard is IMO a good defence, also against other parasites. Soi dogs can be unfriendly to new dogs on the block - we met a pack of three so far on Ko Lanta (still keeping mostly around our house), but they didn't get too close and kept their distance. There are regions where I'd be more wary of walking my dog (i.e. Ao Nang, the packs there are a little too brave). Definitely try to have a good vet near your location (with emergency on-call option), and try to secure local means of travel in advance, otherwise you will get ripped off.
Any advices or questions you may have - feel free to DM me, we are still finding our way around Thailand, but despite it being a big challenge (even finding an accomodation is way bigger pain), it's manageable.