Unfortunately, I get the same errors about eth0 being busy in stage-2 after cherry-picking your PR (and fixing a typo):
Sep 17 09:50:25 patchouli systemd-udevd[1431]: eth0: Failed to rename network interface 2 from 'eth0' to 'enp4s0': Device or resource busy
It seems like the device is remaining as eth0 for the duration of stage-1 even with your changes. Sadly I have no particular idea what could be the problem…
I’m currently not using networkd; perhaps that might help?