The BOI website has a list of approved LTR agents. If you insist on using an agent, then it's one of those you should use.
They are not cheap, over 100,000 baht. I'm not sure why you would want to use one but they're there. You are still going to have to gather and submit all the required documents which is 99% of the work.
I can make a bank statement in 5 minutes using a pdf editor and ChatGPT. If it's not printed and stamped by the bank it is not an authentic document and should not be accepted.
except when you go back to immigration with all of them, they're going to have 2 or 3 new things. Once you get those and return there will be another round of new things. Repeat this until you run out of time and have to leave. They do not want to do DTV extensions.
did I say visa on arrival? I said BORDER PASS. That's what Thai people get if they don't have a passport. Also Thais get visa exempt into Laos with their passport, not visa on arrival like you got.