After warm up temp gauge should stay just below 1/2, if it goes lower at highway speeds then new thermostat is either broken or not installed correctly.
Use 192 or 195degF thermostat, NOT 180
Loss of coolant is plain and simple a leak, you just haven't found it yet.
First, never add coolant to radiator, you add it to the overflow tank, fill to COLD line when engine is cold.
If you want to check the rad that is fine, if it isn't at the top then replace rad cap, one of it's 2 valves is bad.
And check overflow hose, pull it off rad and blow thru it, make sure it isn't clogged, and make sure overflow tank is clean from debris that may settle at the bottom and prevent coolant from flowing back into rad as engine cools off.
When engine warms up coolant expands and rad cap lets it flow out and into overflow tank, when engine cools off rad cap allows coolant to flow back into rad, so even if there was a leak rad would still be full, overflow tank would be empty.