You must be getting up there in age, $900 seems crazy even for Thailand. No doubt the insurance companies and hospitals are doing there best to take advantage of foreigners living here, they got ya over a barrel 😞
I can only speak for 2 immigration offices, and both have young people that will help you with every step, give you the forms that are needed, I mean it’s easier than doing a application for employment in the states!
Hopefully you find an honest agent and they can handle it for you 😂
When I was staying at a friends place, I just took the form filled out and signed, a copy of his thai ID and house book signed and submitted them at immigration with no problems.