This won't be an overnight fix. My first step though is complete. The Synology is now in RAID 10 with 1 hot spare disk. One thing to note; the disks in this Synology are WD reds :(. So even though the storage connects to the Hyper-V host via iSCSI, the VHDX stored on it, which contains the data for our file server is at least not living on a RAID 5 anymore.
Next will be dealing with the local storage situation on the Hyper-V host (the two-drive RAID 1 and the three-drive RAID 5). The only "production" VM storage left on the RAID 5 is our Spiceworks VM, which I'll move to the RAID 1 storage. The other VMs that have their VHDs on the RAID 5 are non-production / testing VMs, so if they're lost, it's not a big deal to just rebuild them.