What is the best way to obtain a 90-day visa for Thailand and how far in advance should I apply?

Actually the best way to do that would be to get a single entry tourist visa from the eVisa website that the 4 thai consulates still open in the US use.

That way you don't need any medical insurance (which IS required if you buy the 90 day Non-O visa from the US at a thai consulate)

Come here get stamped in for 60 days, get your 400K baht transferred into a thai bank account in your name only and go apply for the in country 90 day Non-O visa based on marriage to a thai at the immigration office in the area where you live for 2000baht.

The application will go under review, then you go back to get the Non-O visa and a new 90 day stamp inked in.

Once the funds have been in your thai bank account for 2 months and you have 30 days or less left on that 90 day stamp you can go back and apply for the year extension of stay based on marriage for 1900baht.

That will go under consideration for 30 days then you go back to get the year extension inked in