Can I open a bank account in Thailand with a DTV while taking language courses, or do I need an ED visa?

Forget that idea. You have a 5 year Thai DTV visa. You're Only going to create problems with getting the Ed Visa.

And, just in the remote possibility that you are working illegally in Thailand and are needing a Thai bank account for your Monthly salary... forget it.

You now have the BEST possible Thai Visa.

You can attend any language class on a DTV. However, you cannot open a bank account, as what matters is the type of visa you hold. It has now become very difficult to open an account, even with an ED visa.

Banks will not now open accounts for people on ED visas.

You don't need any kind of visa to take classes in Thailand. You can take Thai language courses if you want.

A bank account will be dependent upon your visa. But you already have a 5 year visa. You would have to find a way to get your existing visa cancelled which can prove challenging. But that's not guaranteed to make any difference anyways. A Thai language visa will only ever give you 90 days at a time to stay in Thailand and many reports that banks are not willing to open accounts for people on Thai language visas. Those reports started long before the current situation with bank accounts.