Safety Wing. For me it's only $40 USD/month, and it works for the $100k COVID coverage. I've only heard good things from other people who have reviewed it.
Awesome, thank you for your help! Just one follow-up question. You said: "expect to be asked why you want to stay." Is it acceptable to say the real answer like "I want to see more of Thailand"? Or should I say something about difficulties with travel? Or does it not really matter?
Update: It turns out the hotel did not file a TM30 for me automatically. But a woman in the TM30 office behind Chiang Mai immigration was able to call my hotel and tell them (in Thai) to go register me right then. Then a few minutes later, after I was successfully registered, she filled out a slip for me, which I had to get photo-copied in the copy shop, and then that copy was included for my extension. No fee or anything, just took a minute to wait for the hotel to actually go register me.
Thanks for the info. I've heard that many Airbnb hosts are running their rentals illegally, and that is why they will not file the TM30. I'm not going to try to argue with him about it. I'd rather just leave him a negative review and find another hotel that will file one—I was going to check out in a couple of days anyway.