Just a quick report. Just did the 30 day extension of my METV (1st entry) in Chiang Mai at the Central Festival Mall. 41 days before my stamp expire.
In and out within 30 minutes. No issues. In addition to Tm7 and Tm30, there was 2 self declarations i had to fill and sign. (My purpose of stay)
TLDR : Answer Summary
A user successfully extended their METV visa for 30 days at the Central Festival Mall in Chiang Mai, reporting a quick process that took 30 minutes and required a few forms including TM7 and TM30. They noted that many people do not utilize the immigration office at Central Festival Mall, despite it being relatively uncrowded.