there are two visas commonly referred to as a retirement visa. One is the non OA, which you typically get in your home country. The OA has an insurance requirement, but you can meet the financial obligations with money with foreign funds in foreign accounts.
The non O requires money in a Thai account. 800,000.
this probably depends on the specific office and maybe even immigration officer. As far as I know, there's no rule against having the funds in separate accounts. But some officers might not want the hassle.
what you need to check is the total fees, including the exchange rate. Look at what Wise charges and look at what Bangkok bank charges and make up your own mind.
This is one reason to eventually go to the income method, if possible. Keep less in a Thai bank. Also, you can set up text and email alerts for every withdrawal, as well as a daily limit on withdrawals. And balance texts and emails every day.