In-practice, leaving the venue after the light show / at 21:00 is guaranteed to leave you in a 1 hour line to leave the expo and arrive at the Osaka Metro Station (夢洲駅) Yumeshima at the East Gate. Not only is this significantly annoying, but the venue is pitch black / unlit. If you are with a group of people and you are not holding onto each other's hands or bags, you will get separated. It is not for the feint of heart. There are no bathroom breaks during the line back to the station. There are no vending machines. If you have children, a stroller, a wheel chair, or any kind of mobility aid, leaving the expo will be dangerous and difficult.
There are not a lot of access options for the Expo. I strongly suggest that you plan on departing the Expo before the crowds/rush (maybe as early as 19:00). Please take it from me – I routinely attend events at Tokyo Big Sight like Comic Market, the largest fan convention on the planet. Unlike Comic Market, you
cannot simply leave the venue walking and get a taxi.