They've delayed the entry gates for non-Thais twice but I think the online registration for vis-exempt entries will change that. They did say the online registration will now start in Dec/Jan
IATA supplies the airline with entry rules, which are displayed on their screens. If the check-in agent wrongly asks you for onward/return ticket proof when you have a visa, ask them to consult their supervisor, and the situation will be quickly resolved.
I have used onward tickets many times. They are valid for 48 hours but the airline do not check as they just want to see the booking. I have used a completely fake ticket as well.
I hope you chose 'long term living expenses in Thailand' with Wise otherwise it will not show as coming from abroad. You used to be able to do a combo with funds for retirement. Some in the bank and some monthly