What are the requirements for re-entering Thailand on a Non-O visa with Covid insurance?

Im a little confused regarding the requirement for entering Thailand with a Non-O visa (Based on over 50). I have a current Non-O visa with re-entry permit isued in Chiang Mai and currently extended for 12 months. I had to travel back to the UK due to my parents berevment and now wish to return to Thailand on Nov 9th. I see I need Covid insurance to re-enter Thailand but have read my passport will only be stamped for the duration of my insurance. As insurance bought within Thailand is so much cheaper will it be possible to get say 3 months insurance before I travel and then extend my stamp once within Thailand at Immigration and buy insurance from within Thailand.

I hope this makes sense.