I just got the DTV soft power April 1st! How long you should apply depends on the embassy you’re applying from, some are more responsive than others. A month in advance is pretty good in general.
I used House of Taste Thai cooking school if you need any recs! I’ve taken a class with them before and they’re super responsive and helpful with this DTV process too
the visa is valid for 5 years and you get a 6 month stamp every time. So you leave every 6 months, you can get it extended though but I’m not sure about that process tho.