Mine took 7 days. Applying from Canada. If you are from one of these countries and have everything in order then it's pretty much approved immediately.
I thought it was the T.8 declaration as well, however for the new evisa system it's another form all together. My local Thai consulate emailed it to me and asked me to sign it and email back to them. I'll try to find it again and upload a copy here.
No you can't just arrive anymore and sort it out at the airport. Go to the Thailand Pass website and apply for visa exempt. There's a few things you need to upload but if you have everything in order then the approval happens pretty fast. There are tutorial videos on YouTube now that walk you through step by step.
From what I was told a couple years back when I was on a Education visa, is that you are not allowed to take the same course again but if the school offered a different language course that you could take that one and extend. Not sure if things have changed because of covid.