Network doesn’t have to be “so fucked” for there to be a loop. It’s an incredibly easy mistake to make.

Unplug everything from it. Plug two good cables with two good devices in and see if it works. Then start plugging everything in one at a time.

Could be possible MSTP is enabled and there is a loop. Not sure if that’s a managed switch or not.

Why do you want to upgrade to cat6? Is gigabit speed not enough?