they mention 15 months which means they got the initial 90 day Non-O visa and then got the year extension (3+12=15 months) so you're correct, they're comin' up to get a new yearly extension (y)
You can just exit the country, go apply for the DTV visa at a thai consulate in another country via the online eVisa website, then come back on the DTV
You do not need to cancel your extension of stay based on marriage (that you got from the immigration office here) before you go get the DTV
if you have an extension based on marriage to a thai or retirement NO, you can do what James said, BUT if it's volunteer or for any other reason you should cancel it
Most people go to the immigration office a week or so BEFORE the cancel date on their paperwork, the office cancels the extension and stamps you in until the date on the paperwork you showed to cancel the extension.
At the offices that won't "early cancel" you have to BUY that 7 day 'application for extension denied' stamp for 1900baht to give you a week to get out of thailand
Otherwise you leave the day you cancel the extension on or run on overstay
Your previous entry/stay history doesn't factor into whether you get approved for a DTV or not, they don't care whether you've been milking ED visas to stay here 😛
Meet the requirements for the visa as posted by the consulate you're applying thru, get the DTV 🙂
most people sign UP for Muay Thai for 6 months to get a FIVE YEAR visa and then never show up even once at the gym 😛
AND
Sadly the "digital nomad" Work-cation category is the most paperwork intensive category to qualify for the DTV for <- and the visa was originally designed for them