@Eve *********
Check with your airline. I’ve been hassled at checkin when having a return flight that was more than 6 months in the future (having a 6 month visa) but after explaining I can extend it in Thailand to 9 months, they made a phone call to someone and let me on the flight.
Also the Thai embassy who issued the visa didn’t say anything about the date of the return flight although they seem to be a lot more picky now.