What are the best options for obtaining an onward ticket while waiting for a Thailand tourist visa?

Train to Laos is 10$ , fly with airasia to Vietnam is 80$

Thai embassies and consulates generally require you to stay physically within the jurisdiction where you applied while your visa is being processed, your running application could be voided. How ever if you only want to stay 60 days and your passport is applicable for visa exempt you could go for 30 days and prolong the visa exempt at the local immigration office for another 30 days . In this case you need an outbound ticket after 30 days (no need to use) for most airline check ins

Do they accept those ‘onward’ tickets ?

We got temp tickets from onwardticket.com. No issues

You won't require anything for your TDAC, the departure date isn't a required field. However your airline might require an onward ticket to allow you to board, best to ask them as some won't accept bus tickets and require flights. If your visa arrives after you enter Thailand you'll need to exit and re-enter to be able to use it