there is no requirement that proof of funds for Non-O based on marriage or year extension based on marriage or retirement come from abroad. That was a made up thing they told you
There aren't agents in those countries that "push paper" (get the visa for you) 🙁
You just make the booking at the consulate you're going to, get the documents together, go apply, pay, wait the processing time and then pick your passport up with the visa in it
there is no "transitioning" to the DTV in thailand 😮
You can't get that visa inside the country
You can only get that visa from a Royal Thai Embassy or a Royal Thai Consulate General in another country 😉
If you're in thailand you have plenty of choices to pick of the thai consulates in nearby countries. Phnom Penh, Jakarta, Taipei, HCMC, Hanoi, Vientiane, Savannakhet, Yangon, etc
you will definitely want to check with your immigration office BEFORE you bounce out and back to get off that OA, if you need to show the funds as having international origin for when you come back to apply for the in country O visa from them.
you need to go to the immigration office and get the hand out for that and the year extension because they are office specific and each one does it a little different
Only bangkok and one other office require you to show proof of funds origin and even they will waive that IF they see you are just getting off an OA and then on to an O visa
You leave when your current stamp is expiring,
come in visa exempt,
get the documents together,
apply for the 90 day Non-O visa for 2000baht,
wait for the under review period,
go back get that inked in to your passport
and
then 2 months later you apply for a year extension for 1900baht