Do I need an onward flight ticket to re-enter Thailand on a tourist basis after having a Non-Imm O visa?

when you fill out the digital entry form they require you to provide the details of your depature unless you have a retirement visar

Onward travel cheap BKK to Singapore

Sadly, that you own a condo doesn't cut any ice with immigrations if you're trying to come in as a "tourist" on a 60 day free stamp (visa exempt) entry.

Also the proof of onward travel is most often an air carrier policy and they require you to show proof you're leaving in the number of days you get stamped in for if you're comin' here without a visa (visa exempt).

If you manage to board your flight without onward travel you are highly unlikely to be questioned by passport control when you stamp into thailand. You do not have a history of extensive visa exempt entries, (as you said you were previously on a Non-O visa/extension)