Congratulations on your DTV and Thank You for your report, but the Thai Embassy in Seoul's website is not particularly precise about what documents are needed for those who are not residents in South Korea and the Thai Embassy in Seoul's verbage seemed to imply that only residents of South Korea can apply there....
But, if one looks at the details for a Thai Tourist E–Visa (no details for DTV E–Visa), then it seems, from your experience, an Entry Stamp/sticker upon arrival into South Korea is acceptable for DTV applications of those not residents in South Korea....
Perhaps try contacting the Thai Consulate in Guangzhou and see if they can cancel your Tourist E–Visa that they issued (unlikely), but you never know....
if your Thai wife is technologically inclined, then perhaps have her pull up the คร.3 (Rose Bordered Marriage Certificate) colour scanned copy in the ThaiD App and download the colour copy from the ThaiD App (you would need a Thai ID Card in order to use/register with the ThaiD App by yourself).
The ThaiD App is from DOPA, Ministry of Interior and has BORA documents and information of Thai citizens, such as Tabien Baan (Thai Household Book), สูติบัตร (Thai Birth Certificate), Thai Marriage Certificate, etc.
That's great that your local Thai Immigration Office at Chiang Mai corrected your Entry Stamp (Stay duration) for you instead of making you go back to your Land Border Entry Point (Poipet / Aranyaprathet) !
Happy New Year! And, how seamless was the interaction with Chiang Mai Thai Immigration Officials (both about correcting your Entry Stamp and doing your 90-Days Report)?
As a DTV Extension will have an Exemption of Re-Entry Permit like the ED Plus Extension, quite curious when someone exits and re-enters on a DTV Extension, will they get stamped in only up to that DTV extension, or will they get a new 180 days?? 🤷♂️🤷♂️🤷♂️