That’s fine if you don’t need one to also apply for your visa. If you do, you’re going to need a ticket rental that’s longer than just a day.
When I said I doubt your airline to Thailand would check the validity of your outgoing ticket I didn’t mean they wouldn’t check that you have one, only that I doubt they would try and log into the airline’s site with your booking to verify fits a legitimate ticket.
So you could get a ticket rental for your visa application then show it to the airline after it’s expired.
The only problem with the onward ticket rental thing is at most I think you can rent a ticket for 2 weeks.
If your visa application ends up taking 3-4+ weeks and they end up checking the ticket after it expires you’ll have a problem.
I think on a 60 day entry you’d be ok with a 2 week window but for a multi entry TR or STV I’ve seen people say they’re waiting 1-2 months in some cases.
I doubt your airline to Thailand would check the validity of your flight out of Thailand but if you’re worried about that then you’d have to get a second rental ticket for a day or two.
The cost of a two week and 1-2 day rental ticket is maybe $20 cheaper than getting a $50 flight to Cambodia and tossing it.