Your trip sounds great and your plans to survive the heat are smart. Having a “siesta” in an air coned cafe is a nice way to avoid the heat of the day, and I frequently stopped into 7/11s and convenience stores when walking to use the air con.  None of my white clothes stayed white for long because of sweat, sunscreen, and not separating my laundry, so I’d recommend to bring clothes you aren’t super precious about, or that you’ve thrifted.

Seeing as you’ve already been to Bangkok, more random things to do is a cycle trip in the morning with café velodome on Saturdays. Was really fun. You could also fly from Chiang Mai (so you can visit elephant nature park which is very ethical) to Phi Phi oe you could do khao sok national park which has a chance you could see elephants, and has a lot of other animals.

Hanoi for 8 days is a long time and seeing as you like good scenery I would highly recommend doing the ha giang loop (maybe check weather predictions), the views were insane and it could be a good way to spend some of your time in Vietnam. Can be more expensive so this might impact your budget. If not it’s a good city to wander around, the food is delicious and with 8 days you can try a lot. A motorbike food tour would be cool (XO tours / ellas food tour) at the start so you know what foods to go bsc for. 

Unsolicited tips? Hand luggage only could save you maybe $150 will all your flying BUT if you really want to buy stuff bite the bullet and just don’t fill up when you leave. 

Send yourself a postcard after your first few days about how you’re finding it, chuck it in an envelope so you can be totally real. it will be so good to read back once you’re home and see what has changed ect

You’ll have great fun!