This is a really good solution. The only issue is that one needs to have access to the video out or IPMI of the server to set the default boot.On ZFS installation systemd-boot is in use for booting not grub. So as it turns out you can define the default boot entry by selecting the entry in the systemd-boot menu and hit "d" for default. Would have been nice to to see this in the wiki-page.
Would be better to have a method to make a default boot setting via SSH etc. in a running Proxmox environment.