Thank you for the info. I have done the marriage extension for the last nine years and found the 'hoops' very easy to get through after the first time which is a bit daunting.
I think your extension will only run to your expiry date. If you extend in April then you've only got 6 months. Get a new passport here in Thailand around Feb 26 (allowing delivery time) and apply for your new extension in April on the new one.