Internet archive has all of them (EU,US&JP) in alphabetical libraries. Also has bios files

Resize array: https://m.youtube.com/watch?v=nV5snitWrBk

Yes you will likely have to rebuild parity. If its write once read many kind of media (for the array) zfs is a lot of overhead and learning. Z snapshots and data sets are possible with a zfs plugin. https://m.youtube.com/watch?v=pLFaDnTVpuM

Cache drive in unraid is used as temp storage before being put in the array. You don’t need a slog.

Unraid is not turenas or proxmox. Tis a different beast you sound like your trying to plan a system that would belong in truenas.