Gauge in middle = good
one year old sensor = good
warmish = ok, VERY hot motor is best
it only takes about 10 to 15 mins for coolant to hit maximum temp, but if the thermostat was stuck open you wouldn't hit full heat at highway speed, therefore be running cool and rich thanks to the ECU thinking you need more fuel to run
So, start the car from cold, let it idle for 5 to 10 mins and see how long the the rad hose that is on the low passenger side takes to get warm. That could diagnose a thermostat.
Other than that, I'd change the oil and change the plugs to new ones. Coupled with a seafoam treatment and fully hot engine (even letting the car idle for 20 mins before the test would be acceptable, not great for the cats but they look ok) I would go for a retest and see how you do.
After that you'll need someone first hand to diagnose further I'm afraid; that's the end of my ideas.