Can I apply for a COVID extension immediately after getting a 30-day extension in Samui, and how does the consideration period work?

you need to check with the Samui immigration office to see how early you can apply for another extension.

BUT

Remember many offices were starting a 60 day covid extension the day you apply for it <- meaning IF you got a new 30 day extension AND you applied for a covid extension right then you'd lose all the days on the other extension.

The under consideration is part of the 60 day covid extension.

Post on a Surat Thani or Samui or one of the island groups down there in the south and ask them to see what people are experiencing.