Residency Certificate is in Jomtien no big deal. Go there after lunch, maybe 2.00pm and you wait only 5-10 minutes. Pickup time is from 4.00pm and another 5 minutes only.
You're right about the age of the bike and it's 1st check only. The "certified service center" at the DLT and those spread all over the City are the same and the processing fee as well. You can bring your bike/ car to any Center with this sign and get Tax and insurance updated in one step. Drop today, pick-up tomorrow. In Phuket the DLT might be located central. here in Pattaya nobody goes to the DLT as it's fkn far outside the city.
no agent, no service. You have to pass a test with your vehicle every year. At those test centres you drop your green book, get the bike tested and pick up the book the next day. It's taxed and insured and tested in one step. You may do it at the DLT, but why driving there if you find branches all over the city?