My most dreaded road trip in China is the expressway between Beijing and Tianjin. Literally every time I've had to go on that thing, there have been major accidents; on one particularly bad occasion I managed by about 3 hours to miss being involved in a 100+ car pileup with multiple fatalities. Crawling to Tianjin ended up taking about 5 hours and I did have to move my business meeting. Seriously, take the train. If you plan your touring in Beijing properly on Sunday, maybe you can position yourself closer to center of town so getting by cab to BJ South Station isn't such a chore. If you are too late to take one of the trains to Tanggu, then just get one to Tianjin station, take the subway/light rail to TEDA, and get a cab the short distance to your hotel. If you can speak some Mandarin, this will be easy for you unless you are carrying large amounts of luggage/business samples.

You can buy a train ticket at any of the zillion little ticket offices scattered all over Beijing, usually little or no wait and just an RMB 5 surcharge for doing so. There's also a ticket office at Beijing Airport though not sure when it closes. This way you can get to BJ South with ticket in hand and just head for your boarding area.