Should I renew my Marriage Extension before leaving Thailand or reapply on a tourist visa later?

You may also consider applying for a Non Immigrant O visa in your own country prior to coming back.

There is some reports that some rural offices will not let you go from tourist visa to non o visa . They make you leave the country and come back on a 30 day visa exempt.

You can only apply when you have 30 days or less left at most immigration offices, so if you plan on leaving before then, you may not have any choice in the matter. You'll also have an under consideration period from when you apply where they may come and want to do a home inspection or talk to your neighbors etc, so you'll need additional time where you cannot leave after you apply.

If you are going to leave before your extension expires and cannot apply for the new 1-year extension, and will not return until after your extension expires, there's no notification you need to make to anyone. Your extension will expire while you're out of the country, and paying for a re-entry permit would be pointless since it's only valid until your extension expires.