I appreciate the advice. Unfortunately with the purchase of the new router/gateway, even if I bought a DOCSIS 3.1 modem and upgraded my service (eventually) to 1 Gbs, the router can't handle NAT over 700 mbs. The DOCSIS 3.1 modem I have, the CM-16 is rated up to 680 Mbs, so I should be over-provisioned with both the modem and router for my 500 Mbs plan. If I'm understanding your post correctly, I should own equipment that exceeds the demands of my ISP's connection.
Your current hardware is great for your speed. Makes sense to hang on what you have for the next couple years.