It's an interesting contradiction between minimising initial build cost vs total build of potentially (in UK costs) £2,500 in disks alone over the years (without replacements) for easier access to bluray content (you mentioned a lower concern on recovery because the BDs acted as backup)
I do hope that disks will become little cheaper over time but the total cost is the reason why I haven't committed fully on this already. I can't justify spending well over 3000€ for convenience of use but I can justify cost of buying a NAS and disks that store data I already have. If adding space will then be just the cost of disk(s) it's easier to accept and pretty much deceive yourself about total cost of the system. Buying a NAS and eight disks will cost more that your monthly budget but you barely notice the price of one disk.
I get not wanting to re-do storage build but given a trigger for this work is the slowing VMs, I think focusing on big storage (when by your own admission you're not sure you'll get around to the work) is the wrong way around.
VMs will not run from same disk space in any scenario. They will have a dedicated mirror set on same computer that runs them, at this point I don't see need for even five VMs but I try to find couple of cheap one TB SSDs for them which should stay well over half empty.
The mirror is faster than the parity method as there's no calculations for parity taking place. The faster speed comes from the stripe within the mirror. 4 disks in R10 using 5400rpm drives is still north of 400MB/s in R10. This exceeds your network speed by ~4X if you're running 1GE.
I don't work with big files over the network, I only store them there. And when I copy large amount of data to NAS I don't need to watch it progressing so it doesn't matter if it takes an hour or two. So while speed is nice it's not essential.
Uplifting the core of the system leaves everything else intact and gives you more CPU power for the VMs. Splitting the storage from the compute leaves things intact and you just move the VMs to the new system.
While I don't think it matters now it may well be valid point at the end of the systems life cycle and something I will consider.
If you're running a 4 disk R6 array, adding to extend to 5 disk and higher is easy.
Can you give an example of (non commercial) system that has an easy expansion possible? If for example it's Linux server what file system it requires?
I have to say that at this point it seems that possibility of storing Blu-Rays will bring too big initial cost when comparing the benefits. Most likely scenario for this to happen would be that I find similar server that I have now which can house at least eight 3,5" disks but it would still require sata-controller, SAS disks cost too much more. I would start with four 16TB disks (which seem to cost same as 10TB disks) in RAID 6, it would be in the (far corner of) same ballpark as NAS + two disks.