>I’ve been told I can apply for a 60 days visit family extension without the 400K proof.
Yes you can, assuming you did not apply for that 60 day extension of stay yet
>Will I need a $
*****
0 coverage health insurance to apply for the extension?
No you will not need the health insurance or coverage. The specific requirements to get the 60 day extension of stay based on visiting family depends on the immigration office, but most require:
1. Your passport, copy of the data page, copy of your visa, and copy of your entry stamp, and departure card
2. Your wife's Bluebook, a copy of her first bluebook page
3. Your wife's ID card, and front and back copy of it
4. Your marriage certificate and a front and back copy of the marriage certificate
5. 1,900 THB