Can being self-employed without selling in Thailand help with opening a bank account?

It's not about your business but the documents they require to open a bank account. If they aren't flexible on it, give it up. Working permit aren't available for DTV anyway.

I doubt that would make any difference at all to the banks.

You either have a work permit or you don't and you can't have one on a DTV

The bank doesn't care. The employee you are talking to has a checklist. If you don't have what's on that checklist, then they will not help you. Employees in Thailand are not allowed to think, go outside of what they are told, or even question what they are told. Initiative is frowned upon. Your only chance is talking to the bank manager, but then they would have to go against their own policy to give you an account which would make them look wrong, so not likely to happen. Best thing to do is move onto the next bank branch, since each branch manager sets the policy for that bank branch.

Why would that matter for them? Passport right visa