Sorry if I missed that but what's gonna be the OS? I mean, on Linux, you can just use Linux Software RAID which is old but gold and will have better performance than ZFS. Otherwise, there are tri-mode NVMe/SAS/SATA controllers.
Pvt-Snafu
Honestly, I wouldn't go with Storage Spaces. Just unreliable. If you're willing to take the risks, forget about GUI for a proper performance: https://storagespaceswarstories.com/storage-spaces-and-slow-parity-performance/ Also, no to RSTe. I would personally go with a hardware RAID controller on Windows.
Are they add just as any other drives to the RAID controller or hot spares as mentioned? Also, are they the same drives as others?
I think that falls under the homedatacenter category:) Looks very decent though!
If you're looking to use it to run some VMs and containers - Proxmox. If you're going to use it as a NAS - TrueNAS.
If you have money for an SSD system, then why not? Also, there is no "no need to modify me for an eternity system" as such. Backups and periodic verification is a must if you want to keep your data. No matter HDDs or SSDs. Also, RAID 1 with 4 SSDs? That's only 1 SSD usable capacity. Unless you mean RAIDZ1.
Well, that's a tough question but I think no:) Unless you have multiple backups and want to experiment:)
There should be no problem streaming 4k from HDDs as others said. As to SSDs, depends how long are you going to store them without power but I'd say they will be fine for several moths or even more. Again, backups. Also, I'd run CrystalDiskInfo to check drives health: https://www.pcsteps.com/2530-predict-hard-disk-failure-crystal-disk-info/
How much are we talking about? I mean, Backblaze B2 or Wasabi might be a good option. Or hetzner if you're in Europe.
Definitely not motherboard RAID. However, why not MDRAID (old but gold) or ZFS (protection against bit rot plus all the other sweet features like snapshots etc.)?
Is boot drive selected in the boot order? Probably yes, but just to check. You can also try using Starwinds free converter: https://www.starwindsoftware.com/starwind-v2v-converter to convert the disks to raw or qcow2.
What's gonna be the workload? I mean, there is caching of course but you could put some most performance-demanding VMs on NVMe drives in ZFS mirror, some slower VMs on 2x4TB drives in mirror and the rest (file, media server on 8x8TB drives in RAIDZ2).