@Patrick ******
sorry you're not gonna get a "based on my experience" answer outta me 😮
You're gonna hafta slog on over to your immigration office, SHOW them the STV and 90 day entry stamp and ASK them what the requirements are at THAT office for you to get the extension. 🙂 That is how you will know.
Keep in mind the people who sold you the visa at the consulate are NOT thai immigrations, they are Ministry of Foreign Affairs workers, they sell visas (among other things). They have abso-tively, posi-lutely NO clue what's required to get an extension from that visa inside the country
Once you're here and to get an extension you deal with the Royal Thai Police Immigration Division, and they are the ones who set the requirements for extensions inside the country.
That's why I said GO and ask them..