MergerFS + Snapraid is a really nice way to turn ext4 mounts into a single entry point NAS. OpenMediaVault has some plugins for setting this up. Performance wise it will max out the drive of whichever one you are using and you can use cheap mismatched drives.
OMV with SnapRaid+MergerFS in a Proxmox VM. I used an LSI card with PCI passthrough to the VM so it could see the drives. Nightly snapshots of the VMs are very convenient if you ever need to restore/migrate your install.
You may get more bang for your buck by getting a comparable mini PC rather than a NUC. Some ThinkCentre Tiny machines are listed on eBay for less than $100 USD. HP and other manufacturers sell their own versions as well.
Do you have an auxiliary power connector hooked up to the HBA? Here is the manual.