Do I need 90 days of COVID insurance for a 60-day visa and applying for a Non-O visa in Thailand?

you need to be covered by the covid policy for the duration of time you'll get stamped in

SO

in theory you only need 60 days coverage because that's all you get stamped in for BUT some consulates want to see 90 days worth of coverage (because they know you can get a 30 day extension) I'd say get the 90 day policy just to be safe..

You won't need insurance once you apply for the Non-O visa based on marriage as neither the 90 day Non-O in country visa nor the year extension require health insurance