A couple of tangentially-relevant things:
Putting zfs on a debian-12 template will require that that template actually uses a debian kernel (or a very old (6.1) fedora one).
It would be nice if you could select debian kernels from the menus that let you select which kernel a VM uses, just like you can with fedora kernels; instead I have to build the kernel into the template, and every template I clone from one like that has to copy that extra stuff.
For some reason if a VM with ZFS installed on it also has network access (via sys-firewall), it won’t work, as in the VM shuts down immediately on startup. I didn’t investigate too much; it could easily be something stupid I did. Or it could be an incompatibility between ZFS and the qubes-network code that has to be installed in a VM in order to connect to sys-firewall.