What are my options after my education visa expired while I was unaware and how can I address the overstay fines?

Just curious why the school isn't giving you paperwork stating you're still studying so you could pay the overstay fine and apply for a new extension based on education? Is your term over with them or what?

The US consulate doesn't issue the letter and hasn't since last Oct or so.

There is no "amnesty visa" so let's get the terminology right for you. You want a 60 day extension because of covid.

You take paperwork from your school stating you stopped studying on xxx date, the immigration office will cancel your current extension (even though you let your stamp run out it wasn't cancelled officially) you will pay 500 baht a day for every day from the date your stamp ran out until you apply for the covid extension.

If you already have the paperwork from the uni, I'd just go back to the immigration office today and apply for the covid extension. Today is the 17th day since your extension expired so you would own 8500baht overstay and pay the new 1900baht covid extension fee.