I went to CM immigration today to extend my visa based on marriage (60 days). Was on a Non-b, then 2 covid visas which runs out early March. They said if I change the visa to a 60 family extension early I would lose my time beforehand. So for CM, go close to your visa expiration date otherwise you'll lose time. They also did not provide a list of what is needed for the family 60 day extension.
TLDR : Answer Summary
A user shared their experience at Chiang Mai immigration when trying to extend their visa based on marriage. They learned that applying for a new visa type (60-day family extension) early means you lose the remaining time on your current visa. It's advised to go to immigration close to your visa expiration date to avoid losing days. The immigration office did not provide a list of required documents for the family extension.