Thank you Todd!
I'm a US citizen who recently applied for a Thailand Tourist Visa while I was in South Korea. In regards to additional documentation beyond what is asked for on their eVisa portal, they emailed me a couple days after submitting the application requesting flight tickets leaving Thailand and bank statement of the last 3 months showing ending balance no less than
*****
0 won. After emailing that to them, they emailed me again requesting a photo of my K-ETA entry stamp on my passport for South Korea and hotel confirmation for the full 60 days of my stay in Thailand.
I wish this was added to their eVisa application saving a week of back and forth emailing of documents but it is what it is plus I much prefer this than having to go physically to the consulate. If you're applying from South Korea, I'd recommend doing so at least 2 weeks before your flight to Thailand since they will definitely need additional information.