You've done one so far? I bet you can wait until year 29 then crank out 6 more.
If he manages to survive to see these through, I'd like to read a post-mortem on why it took as long as it did. For all of it's moving parts, this is still a single story and the framework ought to have been in place before GoT was even written. So many story threads does add a degree of difficulty, but there are storyboarding techniques to keep track of such things and ensure they are both consistent with the rest of the story and intersect back with the main line one or more times. It seems very much like he's winging it without a really clear plan, as if he was expecting the story to write itself once he started telling it. That might work for simpler stories, but not this.