Btw, if you book a flight with Emirates you can change dates for up to 24 months free of charge. In the end buying a return ticket in the first place would be your cheapest option!
With the stv you'll have the problem that you need proof of accommodation payment for the duration of stay. I recommend 60 day tourist visa+30 days extension for 1900 bht.
I've never been asked for an onward ticket, neither at embarkation nor disembarkation!
Not even when I traveled to sandbox Phuket.
When applying for the visa and Thailandpass it is sufficient to just put in the date, time and flight number out of the country. That you can google. Just search for example, Emirates BKK to MAD + date flight number!
I'm on a 60 day tourist visa. In order to get a five year renewal I need a 90 day non immigrant or a one year non immigrant. Since the non immigrant ED visa is the easiest to get inside Thailand, this option was offered to me.