my first Covid extension started from the end of my tourist visa, and my second Covid extension started from the end of my first Covid extension. At each application my passport would be stamped “under consideration “ and to return by a certain date and the second extension I was asked to return days after my first extension expired, so in reality I was asked to overstay my extension. So it doesn’t always start from the day you apply for the extension.
It depends on who you are dealing with and how they interpret the rules. I did both my visa and Covid extensions at the Nonthaburi office.
I believe you can get a waiver, but not having TEFL or similar doesn’t go in your favour and will depend on who you’re dealing with, Thailand is very erratic when it comes to rules and regulations.