I know he has a lot of fans, but I personally couldn't enjoy the work. I wonder if the books didn't so much jump the shark as the people reading them have gotten older and developed higher expecations. With the quality of Trek books having gotten so much better what once seemed top of the line became kind of average. I was able to push through the first four books because the overall plot was interesting even if I disliked the execution, but now having read things like Vanguard I don't know if I would be able to make it through something like that again.
I haven't read the first few books in a long time, but I do kind of have to wonder if this some of what you say isn't true, at least for me. When the books first started, we hadn't had anything like them before, so it was something new and unique. But by the time we started reaching the last few books, we had the DS9 Relaunch, Vanguard, Titan, and the first couple Kirsten Beyer Voy-R books. I don't about anybody else but I think I probably enjoy those overall more than I ever did NF.