Organisation and planning is essential for a trouble free no stress holiday.... plan to leave Thailand well within the 30 day visa limit...... or have an ongoing plan... and the minute you think you just might get away with something.......... usually you dont..................

seems to me you are working it out correctly - but rather than wait until the homeward leg what about if your plane lands at 7.10 then you have a 10 min walk to baggage collection then another 10 waiting for said bags etc etc then go for a cup of coffed mouch round the shops and join the queues for immigration at 9.30 making sure you dont go through until 9.55 just to be on the safe side - you then know for sure that you are fine - another way is depending on where you are visiting to cross the border whilst there - for instance I did a day trip into myanamar just into a village across a bridge and you have to leave thailand and re-enter hence renewing you visa by 15-30 days dependant on which border crossing you use