The ZFS is not a device type for incus profile device add, hence the error you are getting. You should use disk instead.
You can create a dataset for the use of Incus in your existing ZFS pool called ext_storage. See Incus existing ZFS pool
Then, you can add that dataset in Incus as a storage pool, and move images from one pool to another. Or, create images to a specific pool.
incus storage create myincusexternal zfs source=ext_storage/incusexternal
Then, when you launch instances, you would
incus launch images:debian/12/cloud myinstance --storage myincusexternal