I was hoping someone with more recent experience might chime in. But, since you haven't gotten a response, I'll comment. Note: This is based on how Disney Transportation worked when I was a Cast Member about 10 years ago. It's possible it changed...but I seriously doubt it.
Disney Transportation between theme parks is provided until after closing of the last theme park. So, if Magic Kingdom closes at midnight, you'll be able to get a ride back any of the other theme parks after midnight, even if that other park closed at 8pm. This set-up is done to encourage people to use the Park Hopper option.
As the night progresses and fewer people need to get places via Disney Transportation, the wait time for a bus might increase. On rare occasions, you might end up on a van driven by a Cast Member.