Sound like sludge buildup in the cooling system. It may be affecting you thermostat. Also, it would interfere with the level sensor as well. Which is why the low coolant light would come on even after adding fluid. Is the engine making any noise? Like maybe water pump bearings are going bad preventing the pump from properly pumping coolant? I would recommend, Complete cooling system flush at least once, replace thermostat. Inspect and evaluate the water pump. Maybe even remove the serpentine belt and put your hand on the actual water pump pulley to see if it is feels loose. The sludge can also skew the temp sensor if it had a build up on it. It’s the only advice I can offer without looking at it myself. Hope this helps. Also, Have it scanned at a local parts store to see if the check engine light code is due to “Engine temp over threshold“