I also agree with your assessment and course of action. In my experience 850s are not prone to air pockets or needing "burping" of the cooling system. Luckily the thermostat is easy to access on these cars (and the ECT sensor is right there too).

Twice I've seen ECT sensors fail but both times they failed such that the temp gauge spikes to the red (scaring the crap out of the driver until they realize the real problem). You may find you don't need a new sensor, but it's good to have one handy.

One more thought - check that thermostat box to be sure you're getting a new o-ring seal. Last time I did one I found out that they now sell the seal separately.

Best,
Will