Thank you all of your input, I thought those 77W and 748 also have flat bed as long as 81 inchies.
Perhaps I can consider DUS-PEK as well, (although only 3 weekly)

for FRA-PEK, it is more flexible for me in terms of choice of flights,
I generally prefer night flights for ex-EUR and red-eye for ex-Asia.
In that case, only PEK-FRA suits me.

How about lounge experience, any comments on PEK, CTU and SZX?
I would probably consider PEK or CTU for connecting flights rather than SZX