Should I open another bank account for visa renewal after getting my O marriage visa in Thailand?

having a second account is a good idea

Yes i have been with Bangkok bank since 1988 just opened accounts at two more banks. Think its advisable nowadays not to put all your eggs in one basket

Is your 400k in that bank now ? Did you open it on a tourist visa?

Never hurts to spread your money around. Your financial advisor would recommend it.

I've used kasikorn, apparently very reliable.

Bangkok Bank is probably the worse bank in Thailand right now for foreigners.

Yes, I suggest that you open an account with another Thai bank because the bangkok Bank is not helpful with Immigratiion matters.

That's up to you. Just know your bank account could get locked at any moment forcing you to go into the bank with all your documents and potentially waiting weeks for it to be unlocked. And when you want an extension from immigration, if the money for that extension is not in your account for 4 months when you ask for the letter, they will require you to freeze the funds for an additional 4 months from the day you get the letter.