I also applied for a 60 day Thailand tourist visa while in Korea in December 2023. I have a US passport. There was never any question to answer about my finances. No question about onward flight. So, I suppose it is different if you are a US citizen. Just want to also make you aware that I completed my application on my first day in Korea. My Korean holiday was 4 days. I was long gone from Korea before they issued my visa 3 weeks later. I also had to arrive in Thailand using the 30 day visa waiver. I did use my 60 day tourist visa a few months later.
Best to bring as much cash as possible. Some bank cards charge as much as $55.00 USD for cash advance of a few hundred dollars. I used my money to prove it. Leave that card in your wallet and only use for extreme emergencies.