I had this issue with Comcast's Xfinity and my solution was to clone the Mac address of a desktop on the Router.
I tried several desktops/laptops and several routers. All the desktops/laptops got just about full speed. All the routers where pinned at 93.78 or 31.23 for several tests. Cloning the Mac address on the routers that could let the router and devices connected get the full plan speed.
I've got 10 12tb Seagate EXOS drives in operation right now and have also run small capacity (2-4tb) WD Red and blue and Seagate Barracuda drives. For ssds I run Samsung 870 evos.
I'm running cat8 between my modem, router and main switch. The runs to the satellite mesh units are also cat8. I don't want to ever pull wire to those spots again. I don't regret it one bit. Everything else is cat6 with a couple 5e runs
I've got six 2tb barracudas with 200k+ hrs and three with 80k+ I've only lost one barracuda over the years.
I've had nothing but bad luck with "recertified" drives or whatever Amazon calls them. One of the five drives I got off Amazon is still working, and that seller no longer exists. Server Parts Deals has been amazing for used drives. There are also discord and Facebook groups for tracking sellers and stuff.
Been considering building a mini lack rack to put on top of my lack rack to hold my mini PCs.
I'm also having a very similar issue. Desktop stopped working over Ethernet, cable works on another desktop as well as a laptop. It usually gets an ipv6 address, even though ipv6 is disabled at the router.