What I do for manipulating VM root disks (although it’s slow and messy) is to boot from an Ubuntu 24.04 installer ISO image, and then once the agent is running, ignore the installer and instead “incus exec” into it to do whatever you like to the VM’s root disk.
As far as I can see, a VM root disk and a block “storage volume” are two different, non-interchangeable concepts in incus (*). Hence you can’t just attach a storage volume, prepare it the way you want, and then get a VM to boot from it.
(*) And there’s now a third type, dependent storage volumes.