CMBob

I would guess you could get the RT-PCR test at any hospital. I know you can get it done at Chiangmai RAM hospital......3,500 baht (with results later in the day if you're tested in the morning or results the next morning if tested in the afternoon). I understand from reports that the cost at other hospitals can be as low as 2,700 baht. If you arrived on a Monday, I'd think the following Friday is only 4 days away, not 5.

Both I and friends got our extensions at the Immigration Office near the airport. I have no clue if the satellite office at Promenada does any extensions (haven't been out there in years).

Somewhat off topic.....but just a week ago CM Immigration required a copy of the TM30 when friends got a 30-day extension to a visa exempt entry. I personally was required to produce a copy when obtaining an annual extension based on retirement earlier this month. Don't know whether its required for a covid extension but I'd guess it is required.