What should I do if my Non-Immigrant O visa based on marriage to a Thai expires while I'm outside the country?

Depending on frequency of travel the APEC card can be useful for this.

If you travel extensively you might want to think about getting a year long, multi-entry Non-O visa from a thai consulate either in Australia or nearby thailand (like at the consulate in Savannakhet Lao), that visa is good for a year, allows unlimted entry/exit, and every entry gets a 90 day stamp. When that runs out you go to the border, stamp out of thailand, in and out of another country and right back into thailand to get a new 90 day stamp. If you travel a LOT that is a good "fit" visa wise for you because you don't interact with the immigration office in your area at all, no extension to be here to renew, etc

You can enter on a visa exempt BUT you can't renew anything because if it's after the expiration of your extension you lost it

What I mean is there is NO leeway at all

If you're out of the country when your current extension expires you're done. You start all over from nothing, getting the 90 day Non-O visa and then the year extension all over again.

You can apply for a new yearly extension when you have 30 days or less left on your current stamp at most offices. Extensions based on marriage to a thai get an under consideration stamp (from 3 weeks to a month) then you go back to get the year inked in.