The proposed setup sounds good. For the migration, it may be best to first move to Incus without changing anything else. That’s because you can get the exact same Incus version on both Ubuntu and Debian when using GitHub - zabbly/incus: Incus package repository.
Once that’s done, you can then make a backup of /var/lib/incus (after stopping all instances and Incus itself), reinstall the system and restore /var/lib/incus.
That should then get you onto the exact same Incus setup but now on Debian.
Lastly, plug the two new drives and add them as a new pool to Incus, then use incus move and incus storage move to relocate everything from the old pool(s) to the new pool. When done, delete the old pool(s) and remove the disks.
You could also do the storage re-shuffle before backing up /var/lib/incus and reinstalling the system if you prefer.