root disk formatted with ZFS
Solution 1:
Since Eoan Ermine (Ubuntu 19.10), Ubuntu installer for desktop only can use ZFS as root file-system but is still experimental.
The current LTS, Focal Fossa (Ubuntu 20.04) provides only a mono-disk installation, I do not know the difference with a server installer, but this is the only Ubuntu way to install on a root ZFS.
However, OpenZfs provide a way to install Ubuntu on ZFS root:
https://openzfs.github.io/openzfs-docs/Getting%20Started/Ubuntu/index.html#root-on-zfs