The reservoir is slightly above the recommended fill level.

It can't be my thermostat, because I have put in two different, new thermostats, both with the exact same results.

My suspicion is the radiator (not a high enough flow rate through it) and so I will replace that this weekend. Then I'll refill with the exact right mixture coolant/antifreeze, in the right amount, fill with the best procedure (I did use your method of the top hose as that does work well), and hope for the best.