You can add a metadata device to the pool at any time - it's probably faster to add that pool and wait for it to populate than to delete all that data with metadata on spinning rust.
It was a while ago. I think I needed to move the data one directory at a time to get it to move the metadata to the SSD 's.
Since that still would take months I think I simply waited for the next upgrade and moved all the data to a new pool. And deleted the old one.