If I remember correctly, the coolant system is self purging of any air. Unlike an e30 for example, where bleeding the coolant system was required after making any alterations with that oversized plastic philips screw on the top of the expansion tank.

Have you thrown a multi-meter on the temp sensor to see if it is within specification? All you do is unplug the wiring harness and read across the connectors on the sensor. Actual numbers +/- you’re gonna need to source.

Bad thermostat? Once your rig is nearly up to operating temp, there will be a distinct change in temperature at the return hose to the radiator as the thermostat opens. Just don’t lose any fingers on any belts or your fan. Ideally if you had a digital infrared thermometer you could watch the coolant hoses safely. If there is no change, ie one hose is super warm and the other is luke warm, your thermostat isn’t opening.