very good rhetorical questions!
I just moved from RAID 5 on a single enclosure to a RAID 10 split between two enclosures. it’s a bit safer and it’s much faster (~3x), however the TB enclosure has to be three feet away from my iMac to get the speed benefit. I don’t like the noise. I am thinking of investing in a 10g network to keep all of these spinning disks somewhere far far away, and keep the speed

. We use the RAID for our Music and Photos libraries and archiving other stuff. We also keep our Plex disks in one of those enclosures.
Many people are talking about NAS over a 1g network but man that is slow. No point in any RAID setup or single disk faster than 120 MB/s for a NAS on 1g. What happens when you need to move all that data!
I chose HFS+ for my spinning drives except when I need an encrypted volume because HFS+ encrypted is no longer an option.
Time Machine purportedly runs very slow over wifi and Ethernet ortherwise I would use an HDD slot for each family member in the enclosure. I guess there are some things we can do to help… NTFS?
I like your idea of SSD for Time Machine in a dock.