EVA has had a daily direct service (not codeshare) to CNX for some years, be it interrupted by COVID at one point. And they fly to CNX because people like visiting the place.
While not all Immigration offices did, Chiang Mai Immigration implemented the announced changes in 2020 (see link) - only need to file a TM30 on first arrival. I've not only confirmed this directly in person with them, I've filed 90 day reports in person and new extensions of stay using TM30s filed prior to having left the country multiple times.
Chiang Mai Immigration doesnt require a new TM30 whem returning from overseas if returning on the same visa/extension of stay with re-entry permission.
The scam isn't just the medical certificate, it's the whole thing - to get a license, you need to attend the DLT in person to undertake testing and have your picture taken. And a medical certificate (which is required) is only legally issued when you see a doctor in person.