@gringley, thank you so much for posting here!

It's like back to the future. I haven't had to mess with the MTU since the 80s and thought it was a solved problem. The number that worked for me on my Debian Bullseye system was 1491.

$ sudo ip link set eth0 mtu 1491    

I also updated the network settings in the GUI so the interface would be initialized properly (GNOME 3 menu in upper right > Wired Connection > Wired Settings > Interface Gear Icon > Identity > MTU).

Thanks again. I've already joyfully thrown out all of my inadequate workarounds.