[hybridarts]

No you weren't alone and its when you mentioned Sierra that it caught my attention. The Logic user rolled back to Yosemite for now and was hoping to have better luck with El Cap. Not such a problem for Logic but not a recommended OS for PT from what I've read. Might I suggest installing Yosemite? and see if that fixes the graphics issue. Pretty sure 10.10.5 is still supported for 12.8(I'm on 12.5) but a quick check will confirm.

And just read that you had no luck with a Thunderbolt Dock. Which TB dock did you try? I can successfully run a second monitor off a Caldigit TS2 via HDMI or out of the second thunderbolt port with a mini-display to HDMI lead. Another dock that is mentioned on the DUC is the OWC which is popular but I can't speak for it.