[DesertTwang]

I second the recommendation for Dan Miller's Essential Flatpicking series, with one caveat: For someone as accomplished as the OP, the series's pace is painfully slow. Even I found the progress too slow, and that was starting from a blank slate; I had no proficiency on the guitar in any style, and flatpicking still is the only technique I can do. I own all of the books but I didn't stick with them, because the series really is engineered (literally) around the beginning guitar player, and the learning curve is very, very shallow. Dan is an engineer by training, and it shows in the -- sometimes excessive -- thoroughness of how he presents his material. I think the series is excellent for someone who has no distractions that keep them from studying every single day and a lot of patience. Also, every lesson builds on a previous one, and I don't find the Essential Flatpicking series very suitable for someone who wants to jump ahead. I'm not saying the series isn't an excellent guide to flatpicking (it is for sure), all I'm saying is that for someone with distractions like job, family, etc. working through the entire series will likely take many years.

In my mind, the most important aspect that the OP would benefit from is not so much the scales and fretboard stuff, since he probably knows all that, but the emphasis on the technique of flatpicking, and how it relates to rhythm and groove. Those aspects are at the center of Bryan Sutton's Bluegrass Guitar School on artistworks.com, and for the past few years that I've taken his course, I have progressed quite a bit. Bryan's curriculum is much less dependent on sequence, and I could see it being much easier for an already accomplished player to pick and choose the lessons they want to focus on. And as others have pointed out, the fact that you can submit videos to Bryan for feedback is priceless in my opinion. I have had interactions with Bryan from which I learned more than any book could ever teach me, especially when it comes to aspects that simply cannot be conveyed through a book or even a video. It's all about the feedback, and Bryan is the best teacher, hands down, I've ever encountered in my 40+ years of playing various instruments.