Filled it up and it only read 500 on the gauge.
Vapor pressure above a liquid is only based on temperature, not how full it is. 500 psi would correspond to about 32°F. If that’s how cold your tank is, there’s no problem.
FWIW, in a 10 lb tank the pressure will start dropping when there’s about a pound left.
And, weigh it empty and again after it is filled. Should gain 10#. Then it is simple to calculate how much you have left regardless of the gauge.