I paid approx 7500 baht for 90 days of AXA covid insurance in 2020, despite being in a different (cheaper) age band .
I've just been quoted £233 (THB 10760) for a what looks like a proper 120 day insurance, with £5 million of medical cover, including covid and they seem amenable to writing a letter saying it includes covid medical cover exceeding $100,000. (Travel Time Travel insurance).
However, it has been suggested that a 90 day O-visa is not eligible for a 30 day extension, so that part need researching first, before I do an "up to 120 days" itinerary.