I haven't looked into UNRAID at all, but played with FreeNAS a bit and am now running xPenology on my machine. One thing you gain from running xPenology is that it is a port from consumer and prosumer machines. Those machines are known for their OS which is filled with features and has software suites that will work together to get you up and running for what you need. With that said, I'm not sure what UNRAID offers, but in the case of FreeNAS you could accomplish much of the same; however, it took a lot more faffing about it and wasn't all necessarily a GUI driven process. With FreeNAS, for a lot of things you would have to use multiple third party suites, get them to play nice, and then they'd work. In xPenology, there is none of that, it's all there and ready to play nice with itself.
I think another plus for xPenology with your setup is looking at the drives you have put together. So to speak, it's a hodgepodge, it looks like it's a collection of drives you have amassed over time. With xPenology in your use case, they have a RAID structure called SHR and SHR-2. These are fairly equivalent to RAID 5 and RAID 6 respectfuly, but with the benefit of being able to take advantage of different drive sizes in the array. On a RAID 5/6 each drive would utilize only the space equivalent to your smallest drive. Just know the general recommendation now-a-days is to steer away from RAID 5 (SHR) with a shift to RAID 6 (SHR-2) if you want to use those. This is mostly due to the use of larger drives these days. If you lose a drive in a RAID 5 array and have to rebuild your pool with a new drive, the risk of a bad sector on a "good drive" goes up as disk size goes up. At a certain size of drive that risk goes from a risk to likely to happen. At that point you would lose all your data during restructuring when you come across the bad sector. In a RAID 6 array, this bad sector would be equivalent to a second drive down and could be overcome using the additional parity bits the RAID 6 has. I'd suggest reading into it, but that's a very basic generality. Also check out this link to play around with RAID arrays; it has Synology's SHR arrays on it so you can compare and see if there is a benefit with your multiple drive sizes.
https://www.synology.com/en-global/support/RAID_calculator
Good luck with your build!