As AP + BT advice but I would suggest you get a Thai mobile number ASAP (easy to do at most 7-11). If you don't have a lease/ condo already you may be able to pay a few months in advance for your hotel and supply that to the bank and IO as proof of address. (works in CM, other IO may be different). Means you don't have to rush the choice of where to live.
Firstly commiserations on being in this situation. I may be wrong as I am not in the exact same situation but AFAIK for retirement visa the b800k has to be in the Thai account for 3 months after the award then it can go down to b400k until 2 months before renewal when it has to go back up to b800k. Exact dates and amounts are crucial. I'm sure someone else can advise on how to do the b65k monthly on the renewal. It may get you out of the hole to take a loan of b400k for the first 3 months? Good luck.
p.s. Bangkok bank are reportedly getting awkward about the certification of deposits required for immigration to do renewal which (reportedly) is making agents more awkward.
It appears your understanding of English and Thai is limited. Best for you to find a lawyer or agent that can communicate in your mother tongue for advice.
Iirc b800k must show in your account as transferred in from abroad, be in 2 months before and 3 months after application, can only reduce to minimum b400k 3 months after application and must be back up to b800k 3 months before renewal. Anything outside this is why agents charge for IO to 'misread' dates, amounts etc.
Use an agent once, and irregularities will be seen at the next renewal, or if its checked by IO or others at any time and visa deemed to be cancelled.
If UK you cannot use the b6500 monthly pension route as UK government agencies will not provide the required certification.
The interest you 'lose' on b800k is hardly worth worrying about.