Gubei / Simatai - express bus back to Beijing

Anyone here caught an express bus back to Dongzhimen?

I am trying to understand the definite departure times in the afternoon and evening from Simatai / Gubei.

I have seen a photo from parking lot 2 that suggests there is a 21:00 bus, but nobody talks about it anywhere on the internet.

I also want to understand how crowded those busses get. I presume those who arrived by bus in the morning will be going back to Beijing on a bus again. Do you just line up in parking lot 2, 20 minutes before departure?

Is there any way to prebook an express bus to Dongzhimen at around 16:00 or 17:00 on a weekday in April? I am worried I won't be able to get on a bus back to Beijing.

Thanks.