Seems something has changed. The agency that I always use told me last week that they are unable to help with my extension since some changes just happened.
So I went on my own. It took 30 minutes to get an extension done. Zero need for agents now.
Also there was a thread here before about the Certificate of Residency... that you need 10 weeks left on your visa. Not the case. You need a month left... their definition of a month is 5 weeks to get it.
I just got my extension and have a bit under 5 weeks. They were able to move me up so that I get it in around 4 weeks. Then I can come back in the country again after (within 30 days) and renew my license.
This is in Chiang Mai.
TLDR : Answer Summary
The original poster experienced changes in the process of obtaining a visa extension in Chiang Mai, reporting that they successfully completed the extension on their own without the help of an agency. They clarified that the requirement for a Certificate of Residency is now a month left on the visa, which is defined as 5 weeks. Several commenters discussed the lack of necessity for agents for tourist extensions, while noting that the issue with queuing for extensions is mainly associated with non-immigrant visas. Overall, recent changes are highlighted regarding the ease of obtaining extensions.