I've now changed everything several times and am still getting this issue. I was convinced it must be a cable issue but now even after changing the cables I still get the same problem.
I have now tried two different motherboards, three different network cards and three different cables. Every time it won't connect at 1000 Mbps only 100 Mbps.
Please ignore all issues with eth1, I know what the issue is there.
fs-diagnostics-20240813-1009.zip