you don’t prove anything. You need your passport and proof of address to get a TIN from your local revenue office. It’s a free service, I was in and out in 10mins.
Go to your local Immigration Office and ask for the list of requirements for getting a Non O visa based on retirement. Compile the documents on the list.
the Non O outside the Kingdom is the retirement visa. Once in Thailand you do not apply for another visa but instead apply for a 1 year extension of stay.
Once the DTV is issued there is no requirement to attend any booked courses much less an ongoing requirement to a signed up course provided you are not trying to extend in country.