Exactly why I used Issa compass to do the application for me. They charge 18k baht but this includes the embassy application fee. So you only pay a tiny bit more for their service, they know and are up to date on specific rules for each embassy, and if you get rejected they apply again for free.
I was detained for 3 hours at BKK until we figured out a 'solution' ☕️
Definitely get an onward ticket, trip.com has good options with free cancellation.
And id recomend doing VIP entry as well, less questions are asked, if any. Almost guarantees you entry. Some visa agencies offer this. I can send you a contact if you dm me.