I am same opinion as you. It would be difficult foo agents to have contacts in multiple locations. Would have to be where a letter showing a recent balance was fine.
I think different Embassies and Consulates have different requirents. Some need the 500k to have been seeded for example a few months and other it just needs to be there at the time of application. There seems to be no standard like with the Retirement. Agents putting in money for the retirement put it in the applicants own bank account in Thailand. They are with the applicant the whole time at the bank ie they transfer the 800k into your account at the bank. Go to the counter and get a letter from the bank showing the money. Transfer the money back out to there account right away. At no point does the applicant have the opportunity to access those funds. The bank, immigration and agent work together in a specific locations. Doing this for the DTV would be logistically more difficult. Which bank receives the money? Is it a Thai one? Not saying it is impossible but it is more difficult. Are agents working with specific staff at the place of application?
Seen these as well. I suggest if you need that them maybe the visa is not really for them and I would question their right to hold a passport for their own well being hahahah. The financial packet is an interesting one. Agents provide it for those getting the retirement visa. However? It is in your bank for minutes while the bank prints a letter. The agent takes that to their friendly immigration office. Are these agents offering similar at an Embassy or Consulate? Risk all round much higher.
Agents in Pattaya have contacts at Jomtien Immigration. Immigration do not process the issuing of a DTV. The MFA does that at embassies or consulates outside Thailand. Agents cannot really do much for you.