Thanks Gents. You are correct. The "visa" is actually an 'extension'.
Do you know, is the letter from the org essential or can a person just go to immigration to say they are concluding their volunteering? I think a letter is manageable, however if not essential required then obvs going direct is preferable.
Related question. Is Chiang Khong the closest crossing to Chiang Mai? We've only ever been across to Myanmar or over to Vientiane on previous runs, with Myanmar land closed Khong is interesting.
Imvho take the 60 day SETV in the UK online, super easy process. Then extend 30 days when here. Do a short trip out and return visa Exempt for 45 days, renew once 30 days. Repeat. Border bounce is fine but become a pain, make plan and enjoy a peaceful 90 days to start. Cost wise, the cost of a trip out and back will be roughly 1900 BAHT anyhow when you think transport, food, bits and bobs, maybe a room for a night or two. Good Luck, enjoy.