After upgrading a few of my LXC containers to Debian 12, I noticed, that the systemd-networkd-wait-online failed with the error message: systemd-networkd-wait-online[123456]: Timeout occurred while waiting for network connectivity.
After a little internet research I found this forum thread in the Proxmox forum, suggesting, that it might be a perfectly valid solution to just disable the service: systemctl disable systemd-networkd-wait-online.
For me this did the trick, although I did not understand what exactly causes the issue. From the context, I assume it is a specific issue on Debian 12 LXC on Proxmox. Other sources like this one suggests changing the configuration of the service, but for me that sounded off, as my LXC containers only have a single network interface, which is just fine and healthy after boot.