You don't need an agent for anything. I am astounded that these agencies continue to sell their services. All that you need to do 'legally' you can do yourself.
you can either have 800k in bank 65k/month deposit from a 'bank'. I use 'bank' because WISE and my USA Investment Company are not acceptable. Regarding what periods money must be in the bank for your situation I am not sure - I can tell you that I was told as a pperson on a current retirement visa and want to switch to a monthly arrangement. I would need 10 months with a specified form from bank. Don't know where you are located but the volunteer staff at Chiang Mai Immigration are very knowledgeable and have the answers.
If you are over 50. Have 800,000 baht to put in bank or monthly equivalent coming into Thailand from pension then you are ready to go.
Do not fall victim to an agency in Thailand. You can do the simple few pages of paperwork required yourself. Being a veteran has no advantage for relevance.