@Danny *********
I took a GreenBus from Chiang Mai bus station. Took a bus at 8am to Mae Sai, came back on a bus leaving at 3.30pm. I got back to CM at 8pm. It gave me plenty of time to do the boarder bump. Bus fare is around 600thb for a round trip and you can book it online. When you get to Mae Sai there’s plenty of Songtaews that take you to the boarder for a fixed price 15thb (that takes max 15minutes). No questions at the boarder, you just pay 500thb when entering Myanmar, pop in there for literally 10 minutes (or longer if you want to have lunch or shop) and get back to Thailand.