The main difference is that the SETV's enter before date is irrelevant once you enter, whereas it's still important for the METV, since the date is crucial to get the last border run to get 9 months. That's the part a lot have difficulty figuring out.
And I believe, contrary to a SETV, the METV starts the moment it's issued by the embassy (not when you enter). If you get the visa but then enter Thailand 2 months after you get it, you won't be able to get the full 9 months.
If you want to make sure that you have an up-to-date form, get it in paper format at immigration. I never use online forms, since like everything online, they very well might be outdated.
Regarding going from a COVID extension to a Non-Immigrant visa inside the country, do you remember any past reports in the group of it being possible in Chiang Mai?