I just went through this whole thing. A few comments;
1) Yes, it is the thermostat stuck open. I first noticed my temp needle DROPPING from the middle when I was driving back highway speeds from Lake Tahoe.
2) Then I saw it take longer to heat up from cold start. It would reach the middle at city speeds, but as soon as I went on the highway, temp got cold again.
3) I would almost guarantee this is the thermostat stuck open.
4) while you're there, you might as well do the water pump and if you want, the expansion tank. THat is only like 40 bucks for the part (Behr), but you'll probably break the automatic transmission thermonstat, so might as well do that as well.
5) I think you could potentially just do the thermostat by itself, lose minimal coolant, and not have to flush the system. Even the independent quote seems high. You can get OEM thermostat for like $60.
6) Oil temp is very different from coolant temp and generally, in the instrument clusters, the labeling is different. Personally, I would prefer oil temp.
Good luck, OP. but you are on the right track with the thermostat. Everything else would generally cause your car to overheat.