@John ********
if you are married to a Thai national, and can muster the 400K thai baht that is needed for yearly extensions, that's your safest bet. The visa is not connected to you studying in any way. Your stay is secured, as long as you keep 400K in the bank, at least for 2 months before applying for your visa. Each time...
For an ED visa, it varies. It can be from having to get a new extension every 90 days, to having a year at a time. Ranging from low level Thai language courses, to actual university courses (Bachelor or masters for example).
The latter will even give you up to a year of extension of stay after graduation. To give you time to look for a job.
You are allowed to study under any visa/extension of stay. You do not need an ED visa. So I can see how you might utilize a 5 year DTV for that purpose. But.. i do not think requirements for a soft power visa under the DTV scheme align with going to university. There is already a better visa for that (from their perspective). The ED visa. Though from your perspective it might be different. Just leave out uni from your DTV visa then. Find something else that covers such a visa.