Yes you can get the 5 year in Chiang Mai with DTV - Chiang Mai DLT is actually one of the friendlier offices for DTV holders.
If you already have the 2 year temporary license, you get the 5 year when it expires or up to 3 months before expiry. You can't skip the 2 year step, but after that DTV counts as a long term visa for the upgrade. This is confirmed for Chiang Mai recently by other DTV holders.
*DIY process, total cost is about 705 THB, not 3,500:*
1. Do e-learning:
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- 1 hour video for motorbike, 2 hours for car, save QR certificate, valid 180 days
2. Book queue:
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- choose Chiang Mai Area 1 or Mae Hia, service "License Renewal/Upgrade"
3. Documents:
- Passport with DTV and entry stamp
- Residence Certificate from Chiang Mai Immigration - Promenada, valid 30 days - takes about 1 week, free but needs TM30
- Medical certificate from any clinic - 100-200 THB, valid 30 days
- Your current 2 year card
- e-learning certificate printout
4. At DLT: Eye test - color, depth, peripheral, brake reaction - photo - pay 505 THB for 5 year car or 255 THB for motorbike, get card same day. Takes 1-3 hours.
*About the agency 3,500 THB:*
They are just doing the queue booking, taking you to get medical and residence certificate, and translating. You still have to go in person for the eye test and photo. If you are busy it saves time, but it is not required and they cannot bypass the tests.
Since you are about to expire, do it within 1 year after expiry without retest. Over 1 year late you have to do theory again, over 3 years you start over.
Do you need both car and motorbike? That is two separate cards and two e-learning courses.
You could consider reading a book or two before your trip. I’d recommend Unlock Thailand by Isla Rey on Amazon, it includes all the info and insights you need. You should check it out. A very good companion 👌.