What are the current COVID measures and wait times for visa extension at Chaengwattana?

Just be sure you are within 3 business days from your expiration date when you visit CW..

The online appointment system has been down for quite some time. You show up, get a queue number for the section you need and then you wait in the big building that the immigration office is in until it's your turn. They only let 10 people per section in the immigration office area at once.

As far as when to go, there's no telling. I'd say avoid Mondays and Fridays and go after 9AM. Some people have good luck going right after lunch at 1PM though. So you're going to have to decide that on your own.