Coolant system nightmares

Guys I have the worst of the thermostat problems. This is gonna be a long one.

From the beginning I got my car my gauge would sit in the middle at the 190 mark like normal. Eventually it started dropping, mostly during highway driving but letting of gas (manual Tran) would kind of bring it back up sometimes. Meanwhile I was still getting warm air in the cabin. It started throwing p2181 all the time. It would come and go but it pretty much became constant. After failing emissions I decided to finally take it somewhere because I just really didn’t wanna try doing the thermostat and I had already tried the easy sensor on the top without luck (not the correct color one, green if I remember). Anyways the shop did both sensors and the thermostat and it kinda kept it at a constant temp for a day or two but then went back to its old ways. They in turn , put a oem thermostat in. For a while after, it would sit about halfway between the middle(190) and the beginning. They told me that was normal, even tho I told them when I first got it that it sat at 190 and every forum I read said that as well. Forward to the future, a couple days ago I noticed my cars gauge wasn’t reading anything at all, then after a moment looking back again it was right at 190 in the middle. Weird bc it didn’t hit the middle anymore since the new stat. Once I pulled into work and was sitting a minute, I literally saw it drop all the way back down in a matter of seconds. Different reliable shop says I need a new stat and sensors and I’m just really worried I’m going to pay to get all this done and it won’t fix anything again. I have hot air and I feel like it obviously must be warm and just not sending right. However I don’t know which sensor would have to be replaced, and the mechanic is telling me that the scanner is reading temps lower than what it should be so the thermostat might be stuck open a little. I really need some input on this from someone, thank you