How does my non O-A retirement visa work regarding entry and insurance requirements in Thailand?

Had non oa too. But you must cross check with Thai immigration internet page, that for the 2nd year, entering again with the given validity of your renewed health insurance, that this is a mandatory Thai local certificate. Not a European health insurance. And some Thai insurance work on your request only 1 month before start of validity. So you can not enter a starting date months in advance. My case AXA Thailand.

If you enter before 23 october 2025, you will stamped in for 12 months or max the validity of your health insurance (need the special form).

Be aware that you will loose your multi-entry on october 23rd. You need to buy re-entry stamp when leaving on the new 12 months (or validity of your health insurance) stamp.

You’ll get one full year if you have a years worth of health insurance the following year (3rd) you’ll need to show 800k in a Thai bank or monthly foreign deposits of 65k or more for twelve consecutive months

You have a year long multi-entry Non-OA visa that is valid until 23 Oct 2025.

1. 𝗜𝗳 𝗜 𝗮𝗿𝗿𝗶𝘃𝗲 𝗯𝗮𝗰𝗸 𝗳𝗿𝗼𝗺 𝗘𝘂𝗿𝗼𝗽𝗲, 𝗹𝗲𝘁’𝘀 𝘀𝗮𝘆, 𝗢𝗰𝘁𝗼𝗯𝗲𝗿 𝘁𝗵𝗲 𝟱𝘁𝗵 𝟮𝟬𝟮𝟱

As long as you have a new year of the mandatory 100K USD medical insurance and enter before the 23rd of Oct, you'll get stamped in for a whole new year entry stamp (or as long as your insurance is valid for up to a year)

𝗔𝗳𝘁𝗲𝗿 𝘁𝗵𝗮𝘁 , 𝗰𝗼𝘂𝗹𝗱 𝗮𝗽𝗽𝗹𝘆 𝗳𝗼𝗿 𝗻𝗲𝘄 𝗼𝗻𝗲 ,𝗯𝗲𝗶𝗻𝗴 𝗶𝗻 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝘄𝗶𝘁𝗵𝗼𝘂𝘁 𝗯𝗿𝗶𝗻𝗴𝗶𝗻𝗴 𝗺𝗼𝗻𝗲𝘆 𝘁𝗼 𝗺𝘆 𝗧𝗵𝗮𝗶 𝗮𝗰𝗰𝗼𝘂𝗻𝘁? 𝗣𝗿𝗶𝗰𝗲 𝗮𝗽𝗽𝗿𝗼𝘅 ?

Nope you could not. You could go back to your country and by another OA visa from the thai consulate there.

OR

You could meet the financial requirements (either 800K baht banked in a thai bank account in your name only OR 12 months of international transfers of a minimum 65k baht a month) and a new year of the 100K USD mandatory medical insurance and get an "extension of stay" from the immigration office where you live for one year for 1900baht