It's not VoA as that is something completely different. Your visa-exempt entry can be extended for 30 days for 1900 baht and if I was you I would book in a cheap hotel one or two nights before and the hotel will ask for a copy of your passport which they use to register the TM30 online. Tell them you may need a copy (screen shot) of the TM30 registration but if they are uncooperative simply take proof of booking of the hotel you are staying.
Thai immigration want your boarding card for two reasons. Since they suspended the TM6 landing card they want to record the flight you arrived on and the second reason is a throwback to Covid when they needed to know where you sat if anybody developed Covid. Nobody has ever been jailed due to suspicion so stop posting BS. Advance passenger information is collected by airlines and shared with government security agencies to stop people travelling who have been banned from do so. Currently this information is NOT shared with immigration
Absolute rubbish 😃 Immigration only ask for a return ticket if you are selected for secondary screening. It's the airlines that mostly ask. Total BS. There is no way your airline or immigration would know if you used an onward ticket
Last May I obtained a non O based visiting Thai relatives ( there no option for marriage) the financial section says upload bank statement with min £10,000 but this is a typo and you only need £1000. I can't remember if it was one month or 3. The pdf would not upload so I did a screen shot in jpeg and this uploaded straight away. The financial requirements of 40,000 a month or 400,000 is only a requirement for your 12 month extension based on marriage.
It's your airline that may make a fuss - immigration rarely ask. You can buy a temporary ticket for about $14 if the airline deny you boarding. The ticket lasts about 48 hours. onwardticket.com