In regards to route, since you have already made it through Ekok-Mamfe I'm guessing that you should be able to make it.
One suggestion is to NOT take the east route from Franceville to Okoyu in the Congo. It's fun on bikes but sandy and huge ruts from heavy trucks. You would get high centered and there is little traffic to pull you out. Head straight south on the main route instead. There will have been some travellers who have come down there recently so they will be able to comment on the conditions of that road.
If you get your visas figured out, when crossing into Angola, cross at Songololo/Luvo. The road south of Matadi is rough and not recomended. Once you hit the coast, the road has sandy spots so that might be a bit of a challenge, but eventually it gets better as you get closer to Luanda. Southern Angola has some rough roads, but doable.