Hi Max, ref your overheating , I'm no expert with the xt engine but can relate it to engines I have worked on .
1. To check thermostat do as you said , there normally an opening temp marked on the thermostat if not just put in boiling water it should open fully.
2. Make sure the system is bled ,
3. Have you been off road lately , is the rad blocked?
4. Check water fow through rad, flush.
5. Check water pump impeller .
6. Rad caps can cause this if not holding pressure
7. Is the bike running to lean?
8. Any signs of oil in the water or water bubbling?
Sorry for long reply , hope this helps !
Cheers Mark