Can I use a monthly international transfer for the first extension on a Non-OA visa in Thailand?

𝟭𝘀𝘁 𝗲𝘅𝘁𝗲𝗻𝘀𝗶𝗼𝗻 𝗮𝗹𝘄𝗮𝘆𝘀 𝗻𝗲𝗲𝗱 𝘁𝗼 𝗯𝗲 𝟴𝟬𝟬,𝟬𝟬𝟬 𝘀𝗲𝗮𝘀𝗼𝗻𝗲𝗱 𝗳𝘂𝗻𝗱𝘀 𝗺𝗲𝘁𝗵𝗼𝗱? <- that is for people who are getting their first extension on a Non-O visa <- not an OA, and the reason that first extension has to be banked money is because a Non-O visa only gives 90 day entry and you have to be ON the same visa the whole time you're doing monthly transfers to use them for the year extension (you couldn't be transferring the money in while you're here on free entry stamps/tourist visas, then get a Non-O and use 12 months of transfers, they won't allow that..

The OA visa because it gives you a year entry stamp CAN use monthly income method by transfer to get the first extension because you've been on an entry stamp from that visa during the time you've transferred the money in

Non OA multiple can use nearly 2 years If you come back in 1 day before expires with 1 year insurance you will get stamped another year But in that case, if you want to go out, you need re entry permit because that stamp is single entry

If I leave country and re enter during the 1st 12 months with new insurance giving effectively up to 23 months

-> Roughly right. But if saying precisely, you need to enter Thailand again within the Non-OA visa itself validity with a new insurance policy to be stamped for another 365 days (or insurance validity, shorter one). That is not 12 months from the first entry stamp or within the validity of the entry stamp. If you manage well, you can get almost 2 years of stay from one Non-OA visa.

Q. can the 65,000 baht monthly international transfer be used, if I have that evidenced in Thai bank (UK citizen) or does 1st extension always need to be 800,000 seasoned funds method?

-> That is what you need to ask the local immigration office. Some accept monthly and international transfers for 12 months when you apply for the first 1-year extension from a Non-OA entry stamp - i.e. if you have been in Thailand on Non-OA. But some may not.