If you want to fly then just purchase a fake onward ticket, costs about $10. The rules for re-entering on a visa-exempt state you must show 20,000 in Thai cash so whether the IO accepts your Thai banking app balance is up to him/her. I've never been asked to show the money or ticket at immigration in 20 years but I've only ever done 2 land border bounces. Although I have had to show the onward ticket to airlines when coming on a one way ticket - sometimes, not always, as it depends on the agent checking you in.