I wonder why they are asking for this. I'm sure they understand nobody will prepay a booking for 6 months when it's not certain the visa will be approved, and a free cancellation booking will be used, so this requirement makes no sense.
It sounds like you were really studying some thai, so why worry about that? Just say that you want to experience Thailand as much as possible as you have a lot of savings and you don't have to work in the near future.
Yeah, they might stop you if you come several times a year on visa exempt. But if you come and go within 30 days each time and goes back to your home country in between itβs rare to get stopped.