To make this work you will need to move 800k to Thailand before doing in country extensions for the OA.
The non-o dependant receive 90 days and enter, then in country 1st extension will match the OA, not a full year.
It will align correctly when the OA holder extends in country.
You must always have Thai approved health insurance.
You will have to measure the cost of health insurance and it doesn't get cheaper as we age.
The agent option is there, would be somewhat costly yearly to maintain and you get stuck in the agent loop, always need an agent.
This is an email from the embassy and has contacts to ask questions.
Dear Applicant,
Regarding the telex from the Ministry of Foreign Affairs of Thailand on 9 November 2022, the Royal Thai Embassy cannot issue Non/O visa for a trailing spouse if the main visa is Non/O-Retirement or Non/O Volunteer visa. Because it is in the same category.
But we can issue Non/O visa for a trailing spouse/ dependent if the main visa is Non/O-A.