When I returned last December, on a one way ticket, the airline requested a number of documents. Mainly virus related. One of which was a valid visa to enter Thailand.
My visa then, a NON O Multiple Entry, was valid for a couple of months.
A valid visa was also a stated necessity, by the UK Thai embassy to obtain the Thai authorities Certificate of Entry which is/was also required by the airline.
The airline checked everything thoroughly, their only concern was the time from when the test was performed to when the plane was due to depart.
Arriving in Thailand all the paperwork was inspected multiple times at the airport prior to being handed over to the hotel pickup guys.
On previous journeys the one way ticket was noticed occasionally but after drawing attention to the valid visa in my passport there were no problems.
Since returning I have extended my visa and obtained "permission to stay" for 12 months. It is a single entry, which means if you wish to leave Thailand and retain you 12 months "permission to stay" a "Re-Entry" permit must be obtained, prior to leaving Thailand, either at your local TIO or at the airport.
In addition, you are required to notify your local TIO every 90 days.
It appears that you are able to extend your initial "permission to stay" as long you meet the original NON O visa conditions. In my case being legally married to a Thai citizen.
But as always, check with your local Thai embassy and airline.
As suggested above by Slick, obtain a Thai NON O single entry visa and extend, if you wish to travel out of Thailand.