Do I need to provide a return flight and accommodation details when applying for a Thai visa?

No you don't have to have one just make the accommodation up

You can get a provisional hold my fair type booking, or Airbnb can cancel later (Check very carefully) Try and have something planned for at least the front few months of your stay even if you cancel later. I do this every time when I went to the airport because im not booking a non refundable flight back until I know when things are approved. The fayre reservation cost £15 and the airport staff did ask to see it. They are looking for intent of stay. Mine asked for 6 months which was starting to get annoying.

I was the same as you. I wasn't going to spend the money until I got approved. So I just gave them my intended arrival date and the info for the flight I would take on that date. Same for the hotel. I wasn't asked for any proof of by the Los Angeles embassy, but my understanding is that some embassies do ask for proof. In that case you will have to bite the bullet and make the expenditure. But you could buy a refundable flight and hotel booking just in case. Do a search in this group for "Scotland" and see if anyone has made a post about that lately.

Where are you applying from?