I have friends in Hua Hin, they mentioned on several occasions that their local immigration office is very helpful. You can probably get your retirement Visas without an Agent. But much can change in 3 years.
If you want to alter between Bangkok and HH, I would think it's better to get the visa in HH.
I assume you have been to Thailand before and been to the various places that are popular with retirees and also decided where to settle.
If yes, it's best to join Facebook groups for these areas to get a feel of the local Expat community and their general advice on the immigration office and agents in that area.
If you haven't been to Thailand, don't worry about Visas. Get on a plane and come here for 1-2 Months to see for yourself if Thailand is the right place for you.
Had a marriage certificate that needed MFA certification.
Step 1) my home country Embassy in Bangkok had to certify the original document first. Call you embassy and ask if they offer that service and what documents they need. Make sure you tell them it's for an MFA certification.
2) have it translated to English by an official translation services.
3) make an appointment with MFA online (they have offices in Bangkok, Chiang Mai, Pattaya, maybe more)
4) hand original document and translation to MFA for certification.
Takes about 2 business days.
Only the original will be chopped with an MFA stamp.
5) went to immigration, provided a copy of MFA certified original, and copy of translation. Original documents were requested for inspection by immigration officer and then immediately handed back to me for next year's use.
Here's the important point: MFA ONLY certify (stamp) the original document, not the translation. And their certification is ONLY to verify that the stamp of your Embassy is genuine. They don't certify the content, nor the translation.
Important. I needed my MFA certificate for Jomtien immigration, I went there before and asked if they require a translation to English and/or Thai. Answer: English only is fine.
So go ask your immigration office first, they may require a Thai translation.
Use their chatbot: First type in that you want to chancel a flight, then the booking reference... eventually you will get a response from a real person. But only via chat, not on the phone