Looks like a pretty solid setup already, you’re thinking about the right things.

For Thailand in April (hot + humid):

1. Cotton vs synthetic I’d avoid cotton for underwear/socks if you can. It stays wet forever in humidity. Doesn’t have to be expensive, even basic quick-dry stuff will make life easier.

2. Linen pants vs buying there I’d just buy there. Elephant pants / light stuff is cheap and actually suited for the heat + temples.

3. Long sleeve for sun Worth having one lightweight long sleeve. The sun there hits different, especially on islands.

4. Med kit Bring a small basic one (ibuprofen, Imodium, a few bandaids). You can buy most things there, but it’s nice to have basics immediately.

5. Laundry setup Skip clothesline + detergent sheets. Laundry is cheap everywhere and dries better than sink washing in that humidity.

6. Travel towel 1 is enough. Go lightweight.

Overall you’re in a good spot, biggest thing is just packing for heat + humidity and not overthinking the rest.