I have the same issue with my router - it supports 1gbps but that’s a total over all ports. That total includes the WAN port. So the max throughout you can get from one port to the internet is half the max - 500 mbps.
Is that really true? With full duplex, you should be able to route 1Gbps in and 1Gbps out simultaneously. Are you sure you aren't just hitting a CPU or ISP bandwidth limit?