It shouldn't be too bad as you are missing the labor day rush and school has already started. I always make sure to check the calandar on SFFT's website though to make sure there are no special deals going on or large events taking place. I've accidently shown up to parks on days where tickets are discounted and I'll never make that mistake again. I've been to SFOT the same weekend and it was slightly less crowded than usual although I can't directly speak from experience for this time of year at SFFT.