Thank you again for all your replies and suggestions. After trying a thunderbolt gigabit ethernet adapter, I have full 1000Mbps speeds again. I can only assume there was some kind of firmware mess-up with the OS update. At least that won’t affect 3rd party hardware.
I can’t be sure if any of the suggestions helped the situation, but I learned a few things, and at least I don’t have to worry with it now. Thanks again. Cheers!

Maybe you should read your own sources before making unsubstantiated claims. If it had been intentional, using an adapter would not have solved the problem.