I'm going to ask my wife to try and purchase the G18 first.
It leaves at 3 and arrives at 8.

This gives me some time to hang out in the morning and gets into Beijing early enough to still get some dinner.

If the G18 is sold out, then maybe G20.
Although G146 is not too bad (even thought its a 3 digit).
It leaves 15 minutes before G18 and arrives 15 minutes after (yes its an extra 30 minutes) but 8:15 back in Beijing is not too bad.