Just remember if the data is important, raid is not backup. Have the media in another place. I have like 40-50 1tb drives I’ve copied stuff too in case everything goes belly up. Restoring won’t be fun but it will be possible.
If it were me personally, I’d get two more 16 tb drives and Raid 6 the whole thing. But that’s only because you’ve got the NAS already. If I just had the drives I’d set up a JBOD and either use TrieNAS, Unraid, or maybe OpenMediaVault.