Can I apply for a non-O (married) visa from outside or inside Thailand?

Either. Get a non O from overseas and you’ll get 90 days. When that’s winding down you apply for the year extension.

Converting inside Thailand is similar, you first get the 90 day visa (2,000 baht) then get the extension when it’s near the end.

Another option if you don’t want to keep (or have) the 400k needed is to use the consulates in Savannakhet in Laos or HCMC in Vietnam where you can apply for a year long Non O. It lasts a year but on entry you get 90 days. You have to border bounce to get your next 90 days.

You can do either. Are you in Thailand now?

You can do either one