You have a massive air bubble somewhere, more than likely. Especially if the temp gauge spikes, bubble is likely right near the sensor.

Either find a way to pressurize the system, dealerships can do this, or go on Amazon and find a giant funnel that you can fill with coolant to push down and hopefully force the air up.

I have this kit and it worked for me: https://www.amazon.com/gp/product/B07NGPK6YL