I believe what you are saying about your friend’s schooling. Really depends on the school and location. For example, tuition is over 1 million baht at my son’s school in BKK (high school)- and that does not include uniforms, lunch, field trips.
Yes though I am referencing POS terminals where I believe it says home currency or Thai Baht (always pick Thai Baht). On ATMs it says with or without conversion, as you say. Same principle.
You can use a foreign-issued credit card, but check whether it has foreign transaction fees. Also, if the terminal asks whether to pay in your home currency or in Thai Baht, always choose Thai Baht, otherwise, you’ll likely get a worse exchange rate.
If you are coming to check out Chiang Mai, you can enter Thailand for up to 60 days without applying for a visa beforehand. It’s a longer answer if you decide you want to live and retire here as more variables come into play.