I've not chased it down anymore since my last update. The valve cover was replaced at the end of last year before a road trip which provided additional improvements, but the data still looked the same. The highest LTFT partial goes is +19% or so. With the warmer weather coming, I've seen it drop to +15%. My only guess is maybe leaky valve train or piston ring(s) as I'm not sure what else to check from a smoke test perspective since the shop checked both sides (intake/exhaust) and both O2 sensors have been replaced. Will be interesting to see what happens when gas stations put the summer blend in their tanks and the weather gets warmer. I once saw it drop to +11% when it was unusually warm out during the winter.
You had mentioned the coolant sensor potentially being a culprit, but I've kinda gotten tired of throwing parts at the car. A leak down test would tell me if the valve train is leaking so depending what I see this summer I may decide to do that. It runs well enough for me...at least compared to what I was working with when I started this thread.
Will add, am teasing the idea of reinstalling my runner flaps next time I get the valves cleaned to see if that has an impact. Throttle response and exhaust notes are better with them out, but if I get a cleaner or more complete combustion with the flaps in, then the other stuff doesn't matter as much.