If you have an OA visa expiring in December all you need do is step out of Thailand a couple of weeks b4 expiry date then come back in and you will be stamped back in for another year. No need to tran 800K.
You will need to have another 12 months med insurance organised and possibly a re entry permit if your OA is not multi entry.
We should all f off to Vietnam. They have just reintroduced their 90-day visa which you can extend and extend and extend...no immigration to deal with, no 90 day reporting. Its pretty good.