144h visa free tranist FRA -> PEK ->BKK with valid tourist visa

Hello,

I am a German/US passport holder and I will be traveling from Frankfurt to Beijing (arrival on the 23rd) and then plan to fly to Bangkok on the 28th. Would I be eligible for a 144h visa free transit in Beijing? I am asking because I read at a few places that the direction of travel might also count, since I am effectively flying back towards Bangkok from Beijing.

Also I have a valid single entry tourist visa in my German passport, which I want to save for next month to travel around China. For the above visit I just want to meet with a few friends in Beijing, but not leave the city. Can I enjoy the visa free transit first, and save my tourist visa for later? Or maybe just enter with my US passport and apply for a 144h visa free transit, and the second time with the tourist visa in my German passport?

Thank you for any help or suggestions.