Read the news from the last year about how many problems people are having with frozen accounts.
In general it's unwise to put a lot of money in Thai banks, the frozen accounts fiasco has highlighted that. Look into putting your money into Interactive Brokers - they let you deposit cash in different currencies and even pay interest. *You don't need to buy stock. Or look for a similar world-wide financial institution.
I've suspected that besides proof of a matching Thai ph#, since they always request passport for ID and to photocopy multiple times - as soon as they see no current long term visa, the result would be making you close the account. ☹️
Mail was supposedly the only method then too. You might want to check your statement unless you've confirmed with the embassy that they no longer offer in person services.