Thai immigration may accept a one way ticket. But not all airport ticket agents are experts in Thai visa regulations. They get told that passengers need an onward ticket and they gonna enforce that.
Go see a visa agent. They know exactly what to do. Also they can fix you up to get a no hassle drivers license with a motorcycle endorsement. You can do what you want on your own but how many visits will you make first?
Go to the visa agent on Soi6 Pattaya and ask. They run a van load of clients to a local driver license office 2 or 3 times a week. The process to get Thai license is simple. I’m sure other visa agents will help out also.