exactly. Why the airlines even care is beyond me. They are not Thai immigration. On another point, I have never been asked for a return ticket except during COVID. I think this has become a holdover practice from COVID and the airlines are slow to change. It's hard too keep up with every countries visa requirements so they assume the worst case scenario. Pre COVID I went to Europe and was never asked for return ticket.
I have only during COVID. However, if you get the TR visa you have to upload you tickets so immigration already assumes you have a return ticket. That's why they don't ask. The visa exempt entry could be different.
How long do you want to stay? It cost me $40 for a TR visa for 60 days but could get 45 days on arrival for free. Both can be extended for 30 days for 1900 baht. Up to you.