svm root volume hardly contains any concerning space, so leave it alone. The primary purpose of StorageGRID (FabricPool) is to reduce storage footprints and costs associated with Active "user--data" on the high-performance/SSD local tiers. If a catastrophic disaster destroys the local tier, a new environment cannot be created using the data on the cloud tier b'cos Access control lists (ACLs), directory structures, and metadata (WAFL) always stay on the local tier. This is not a Data Protection or DR feature, for data protection/DR, you will need to setup snapmirror/vault.
Also, I see you have mentioned the word 'migrating' , I guess you meant "tiering" inactive data to StorageGRID.