2007 SE.. Ex-leaser getting back into the swing of ownership.. in my 30's, haven't had to work on a car (95 Maxima) since high school..

It's about 33 degree weather.. Car was driving poorly, burning through gas.. Temperature gauge hasn't moved in over a week and that's why I'm here. I'm getting heat though. The radiator fans run, the top hose is hot, the bottom is cold.. I thought the thermostat was stuck open so I replaced it.

Forgot to fill the radiator back up before taking for a ride around the block to see if the temp gauge would move.. I filled the radiator back up and got rid of the bubbles immediately after. The MIL light was on ... I have an OBD2 Wifi module, which I used to learn the code was a P1217 .. "Overheating", obviously because it was running without coolant, but the light cleared though after fill. Car's driving a little better, but I wouldn't say 100% just yet.. Tightened the hose to the thermostat after spotting a leak..

Second reason why I'm here.. Throuh reported the coolant temp at 230 degrees F so far in the past 40mi, short distances in the residential area since Sunday.. Fans kick on around 210 deg and I guess into overdrive around 220.. Idles around 220 I feel so far, varied between 215 and 226.. I read I can operate safely to 240.. but IDK where any of this normally reads on my gauge because it's not working..

Should I be concerned about the 230 or stop bein a baby? I want to take it for a spin on the highway, you know, 70mph or so for a while because I travel interstate yearly, but I don't wanna get stuck out there on the highway.. The temp gauge still doesn't work so I'm just relying on my phone.

At what degree temp is "overheating"? so that I can know to pull over early if necessary?

Any ideas what I can do to get my dash and my car back to normal?
Should I be looking at the sending unit, or the ECT sensor or both? From what I read the sensor is needed to control functions like fuel injection, transmisions shifting, etc.. which I feel would explain the car's performance not 100%..
The needles move on my dash, even the temp gauge (as soon as you switch ignition to ON) so it's not like the needle's completely frozen on the dash..

I picked up a voltmeter and am looking to test the sensor