Based on my 16 years of doing extensions for both marriage and retirement I have never had to endure more than 4 hours at an immigration office......and I have used 3 different ones, Chiang Mai, Nan, and Surin.
NOTE: All the lengthy times were at the CM office back during their busier era (later years at the old building and at the Promenada) before the new building.
I've done extensions at Nan and Surin numerous times and it's never been more than an hour (process and queue times).
So in summary, four hours is hardly an issue when I've been stuck in BKK traffic that long before. What's an agent costs 20k-30k? No thanks, 4 hours (at the most) of my time in an immigration office is no big deal to me, I can surf on my phone to easily pass the time. Yes, I understand that I also use my valuable time getting all the paperwork completed.. It's not rocket science.