You said you have a Cat6 Cable coming and also it is the same cable that the C7 used. Did you try the cable that came with the C8?
By chance did you check the Firewalla to see if it is disconnecting the C8 for something it is detecting? I have heard of firewalls disconnecting devices when they are suspected of being a bad actor in some way. Doesn't mean it is, but just that the firewall device thinks it is.
The comment you made about hooking up a switch between the C8 and the Eero could be a good troubleshooting step to validate that link is still occurring and isn't lost for some reason.
You really shouldn't discount the folks on this forum. All of the folks trying to help you are technology professional like you. You need to realize you aren't dealing with a Linux server, but a ARM processor running a 10/100 network phy that may be running Linux, or some Unix variant with Java. Don't discount other experience simply because your different experience on different hardware says they maybe wrong.