I get cash from SCB ATM with my foreign bank debit card. I always use the "continue without conversion" button. I check the Visa exchange rate online before I make withdrawals and it's correct every time. SCB charges 220b fee, not 250b fee. My bank refunds the 220b fee. The difference in the Visa rate and spot rate is about .10 THB, not bad. The Visa rate changes around noon BKK time, so I watch the spot rate, and if it drops overnight, I will go get cash before Visa lowers their rate. My Chase debit card uses the Visa rate, not SCB’s rate. I use my foreign bank CC for everyday purchases and I always pay in THB, which means the Visa rate will apply with no conversion fees.
Yes, it's quite expensive to let the Thai banks and the retail stores do the conversions.