When should I apply for a 90-day Non-O marriage visa after entering Thailand visa exempt?

As Ellie stated Chiang Mai requires you have a minimum of 15 business days (M-F not counting weekends or holidays) left on your current stamp to apply for the in country Non-O visa thru them

As you pointed out that's roughly 21 calendar days (give or take) so you need to see how many days you have left on your current stamp, take the one 30 day extension if you need it and then apply,

Have you had one already this year ?

You can apply for a 30-day tourism extension first, then apply for an initial in-country Non-O visa.

Be careful to count remaining days if you want to get a 30-day extension. Chiang Mai has a policy of remaining days of stamp that is actually 15 WORKING days, and there are several holidays in December - January.