go to onward ticket .com (I think there are a few similar) and you buy a part reserved seat to give you a ticket/boarding pass as evidence you are not just coming to stay in Thailand longer than your 30 day visa on arrival (if they ask, which they don’t but now it’s getting busy and sometimes previously they have asked for evidence of ticket to leave) just to be safe - I’ve always done it and my friends run a travel company and always use it around the world as many countries you go to do ask to see a return, so I take there advice especially seen as I travelled a lot and Thailand is one you don’t want to get wrong or questioned because can easily just say no 😂depends how the immigration officer is feeling that day. I’ve heard many stories 😅
Get ‘onward ticket’ to be safe I always have - it’s $10 just book one in 30 days time to Vietnam or something, and if asked you show, but they never asked, have some people I know.