This would be my first port of call.
As others have explained above, although they may physically fit the RJ45 socket on your home router or Nucleus, CAT 7 or 8 cables aren’t designed to work with those devices.
You’re effectively using the wrong cable for the job and the shielded / metal plugs on your CAT 8 cables are potentially causing you unwanted issues on your analog audio devices (ie. ground loops, the cable acting as an aerial if only connected at one end on poorly designed kit) as these devices weren’t designed to be used with shielded Ethernet cables.
By contrast the plastic RJ45 connectors on a bog-standard standard CAT 5e or CAT 6 cable ensure the units are isolated from each other as intended / designed.
CAT 5e or CAT 6 cables (not CAT 6e which uses shielded connectors) are what you should be using with your home router / Nucleus / streamer. With your current equipment the numbers 7 and 8 don’t mean better, it means a different and potentially incompatible standard that will potentially cause you issues.
Petrabytes of data is reliably transferred over millions of miles of CAT 6 and CAT 5e cables every second, it’s a pretty robust standard and far exceeds the requirements of Roon.