What documents do I need to apply for a Thai visa extension at an immigration office?

A tm30 most likely although might not be the case for a tourist visa but definitely check with someone on that as immigration love stinging farang with their 1600 baht penalties that are supposed to be the landlords fine

Tourism is down so I can’t see them changing anything soon especially now Vietnam is becoming very popular

At the moment it's still 60 days keep an eye on it

IF that happens, apply for a tourist visa before you come and get 60 days. Easier than going to immigration in the middle of your holiday.

I don't think they're going to change it back to 30 days anytime soon. But that's just my take.

You won't be popping into any Thai embassy, as there are none in Thailand. If you need to apply for an extension, you would do that at your local provincial immigration office.

But if they change it back to 30 days, it would be much easier to just apply for the tourist visa online before you went to Thailand. 10 minutes on your computer instead of 1-4 hours at an immigration office and worrying about paperwork and copies and the drive there and back.