Should not need to toggle to anything, all I do is have both eSIM on and sim on and set all calls, data and messages to the thai number yet still have access to my au number
Just put a flight and date they don't ask for proof of booking. I have done it multiple times. Go via Sydney thai consulate get Pre-approval online much easier than Canberra plus if you get declined via Sydney you haven't paid so you don't loose your application fee