@Bo *******
I think indeed, the first part of the answer of Brandon is the best. Only open a bank account via an agent and put the 800k฿ (a bit more, not too critical) in that account as international transfer. And then ask an incountry non imm O based on retirement with the 800k฿. After about 60 days you can ask for the year extension, again based on retirement and the 800k฿. Should you also want to visit other countries, also arrange a re-entry permit, so your year extension keeps valid.
I notice the money isn't a big problem but I still find it a pity, when you have enough monthly income, to lock up the 800k฿. So maybe a tip to also start transfering a bit more than 65k฿ every month for 12 months as international transfer to that account.
So for the second year extension, you can use these 12 transfers as prove of income instead of the banked 800k฿. (You need money anyway over here) Just a tip.
I know nowadays there are more and more visas to stay, but I still find doing the year extensions once a year the most convenient. But ofcourse it depends on personal circumstances.
Good luck ! 🙏