Welcome to HU
In regards to the route south, the most recent and reliable info might be on the ground from truckers and buses coming up from the south unless there is indeed an overlander that has just come through there.
In regards to the route east through Franceville, the road all the way to the border with Congo-Brazzaville is either paved or all season gravel. On the Congo-Brazzaville side it is sandy for roughly 75km and then loamy for about 75km.
When I went through last year on a bike, I would guess that it was passable in wet and dry season. I also met a couple in a Land Rover in Luanda that had come through there in March/April so roughly the same time as you are intending.
The one thing that they noted was that in the loamy part of the road, there are deep, deep ruts from the large trucks that ply this route and two foot deep ruts on either side mean that you will be dragging your skid plate for long periods of time. On a bike, it means a lot focus to not pinball off the wall on ether side of the rut.
As for visas, friends of mine that I met up with in Matadi, had attempted to get their Angolan visas in Kinshasa for a bit and then moved onto Matadi where they eventually got them. By the sounds of things, both places are fairly unreliable and time consuming so you can get more news when you get close and pick one based on which you believe will be the easiest.
From there, the best border crossing is back at Songolo where the road on the Angolan side is much better than if you cross at Matadi.
As you pointed out, information get dated pretty quickly and even though info from last year may seem like a long time ago, for parts of West Africa that is actually pretty recent. I'm sure it would be invaluable for others if you were to post your experiences coming down as the most current source of information for coming from the north is...you!