OK, I agree after the thermostat open/close hot water test, it sounds like the ECT is out of calibration & that can affect fuel trim over time.
An acting out fuel injector with a runny nose can mess with fuel trim & may be what your seeing in the intermittent +10% stft spikes, but we don't know what cyl bank the graphs are for. Knowing that would lower your suspect list to 4 injectors.

If you come to suspect a runny nose fuel injector, maybe consider trying a 20oz dose of Techron Concentrate Plus in the tank, at the pump before filling, so to get good mixing & run most of the treated tank out before refilling. If we fill up with Chevron, Texaco, Caltex, or now Shell, all of which also have some PEA in their fuel ad pack, we can raise the treat rate 10X above pump gas alone & often get a tidy up of the fuel injectors, intake valves, piston crowns & combustion chamber heads in one treatment. BTW Advance Auto has a BOGO on Techron this month, but better hurry if you want the special.
You should be able to measure/see in graph form, any improvement if the short term fuel trim spikes lower, or go away after using it.

Right now without seeing how O2 sensor switching range & speed looks, my best wag on the lower mpg is a runny nose fuel injector, along with an out of calibration ECT & maybe lazy switching upstream O2 sensors, all combining to lower your mpg.