Is applying for an extension of stay based on spousal grounds the right approach after entering Thailand on a non-O visa?

You got a 90 day stamp (which is what everyone gets entering on a non-O visa whether it's single or multi-entry)

And

as you found out, ALL the borders are still closed so you're not gonna bounce out and back to get a new 90 days like you used to.

When your 90 day stamp has 30 days or less left on it you have two options;

-Apply for a yearly extension of stay based on marriage to a thai by meeting the financial requirements and submitting the documentation. (you'll get a 30 day under consideration stamp then go back and get the rest of the year extension inked in when that stamp runs out)

OR

-Apply for a 60 day visit thai family extension based on marriage to a thai (this requires no proof of funds) and is issued on the spot.

It is highly unlikely you're going to be able to bounce out and back any time soon. So while you're visa is indeed a multi-entry and valid for a year I'd say you have the two extension options I outlined if you intend to stay longer than this 90 day stamp you got when you entered.