Latency or QoS?

Faster.  Your higher plan isn't faster but is bigger.  It's just a bigger "pipe" to download data.  However, higher plans do have bigger upload pipes, so it may seem like your service is faster, especially with gaming because games upload lots of data, due to the bigger upload pipe.

The bigger pipes may also seem faster because your plan can now fit the constant retransmissions of data due to persistent errors.  You may have had problems all along but instead of troubleshooting the errors with you, Cox just said get a bigger pipe (to easily handle their errors)...and then comes GamerElite, Data Blocks, Complete Care, etc.  The question becomes:  Would you ever fill this bigger download pipe with 1 billion bits?  Probably not.  This is how Cox shills needless plans because a lot of gamers need these bigger upload pipes.

QoS.  QoS is a feature within the router but it's a feature to apply to a specific device.  For example, if you want your Gaming PC to have top priority among all other devices, you'd enter its MAC address (I assume) and set its priority.  I would assume QoS applies to wired devices as well but you'd need to research the User Guide of the specific router.  QoS would be an enhancement...not a fix, unless you wanted to beat your son at gaming, then set your device High and his Low.

Ethernet shouldn't be dropping connections.  Ethernet is why people go through the hassle of hardwiring devices because it's a more dependable, reliable, secure connection.

If Ethernet is dropping, it could be the router (personal LAN) or Cox (WAN); however, Panoramic makes this hard to troubleshoot because it's an all-in-1-device.  For example, if you directly connect your Gaming PC to a standalone modem and simultaneously play a game, stream an UHD movie, stream music, perform a speed test and send a 20 MB email with success...probably the router.  If not, probably either the modem or Cox.

If you're to keep renting the Panoramic...and in addition to PingPlotter...upload an image of your modem logs (signals and events).  Also, inventory any coaxial splitters along your cable...in front of and behind the wall plate, attic, box bolted onto the side of your house, etc.

If we can identify out-of-spec signals at your modem, this would not be "very costly' because Cox cannot charge you to fix their problems; however, Cox already got you for Complete Care, so it wouldn't matter.