I’ve been coming to Thailand twice a year since 2012 I know live here in 13 years I have never been asked if I have a return ticket, and if they do tell them the truth you going to get a 30 day extension, you won’t be refused entry
you need a long term visa if you are over 50 get a retirement visa from your local Thai embassy, don’t worry about making any mistakes they will come back and ask for more info if needed, I did mine in UK and it took about 3 weeks
I think you hit the nail on the head, you straight with them they straight with you, I have never had any problems in Thailand with my marriage visa or my bank,