Hi, I am with Kalasin Immigration.
I look at it this way: as part of your visa requirement you need to have the B800000 in a bank account. Just park it in a reputable bank, fixed deposit and forget about it.
I Have a Bangkok Bank fixed deposit for for the B800000, I just leave it there and forget about it. I show Kalasin Imm once a year.
If you're going to chase interest rates then you have to remember the rules about the time it has to be actually in the bank, ...months before ....months after...too complicated for me.
Also the person with the retirement visa has to open a new account, so they would be bank hopping, a pain.
Extra interest is probably less than B 5000 anyway. ????