for the first 30 extention you need to fill out the tm.7 form. A copy of your photo page of passport, stamp page, and departure card copy. 1 4x6cm photo.
At the Samui immagration, you have to first get the 30day extention and if the situation still allows for 60day covid extension. You can do so 7 days before the 30days is up.
you will receive a certificate plus your covid tests results on paper from the hotel at check out. I was never asked to show them when checking in with Bangkok airways.
no you should be able to apply for the STV through your nearest Royal Thai embassy or consulate when it's made available. No need to use agency or company.