combine SSD and HD

If you did a factory reset, you could create a 1.5 TB RAID-5 array with all 4 disks. I don't recommend it though - you won't see much performance gain, since sequential reads/writes still cut across all four disks.

Your second idea is a better approach - switch to flex-raid, and create additional volumes. RAID-1 for the SSD's is possible; there are two other options you could consider. If you have a good backup for the SSD array you could use spanning RAID-0 (creating 1 512 GB volume using both disks). I don't usually recommend this, since if either SSD fails you lose everything in the volume. But if you know that up front and plan for it, it is viable. You could also create two new volumes, 1 for each SSD.

You likely won't see much (if any) improvement in streaming performance. You should see faster access to small files and and the database itself (since there is no seek time, random i/o is faster).