Related: https://michael.stapelberg.ch/posts/2020-08-09-fiber-link-ho...
As to the author's core complaint (CAT5/6 degrading to 100mbps), I've found that the issue is almost always the termination at the jacks. Reseating the wires with a punchdown tool, or failing that, cutting a bit more off the ends and ensuring that the pair twists run all the way to the punchdowns has been enough to restore 1000mbps negotiation the few times I've had this issue.