Lot to unpack, just a quick answer:

1. Don’t do that, forget the USB hub it’s unreliable, if you absolutely want to go with the USB route get something like a HDD multi bay enclosure: sabrent.com/collections/…/ds-sc4b But if you don’t want to fiddle with things get a ready made solution (qnap, synology, ugreen, etc) you get the hardware and the software, just insert the disks and you are ready to go.
2. If you are doing DIY look into ZFS, or use the solution from the software you are using, Unraid for example.
3. The usual advice is to do the 3-2-1 backup (three copies of your data across two different storage types with one off-site), but entirely depends on you and your data, keep at least a separate copy of your most important things.
4. Sounds good.