The only thing you listed that is required for a non-O based on marriage is proof of funds. None of these other things are required for a non-O visa. You're listing requirements for the non-OA which is not what you are wanting.
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.