Selfhosted

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: https://sabrent.com/collections/hard-drive-accessories/products/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.

If your N100 mini server has an ITX motherboard with several SATA ports then changing the case to something like a Jonsbo N2 might be a solution that works better than USB.

1. There is no "special" benefit to a pre-built NAS. They have convenient software but there is nothing exceptional about them. They're just computers with storage drive slots. Using a bunch of external drives via a USB hub would be fine. But is that your only expansion option on the system you have? Access speeds via USB, especially if using a hub, won't be ideal. It'll certainly work, though. You can also get enclosures to put full size HDDs in, which can connect to an existing system.

2. RAID is still the way to go, but since you don't need much storage, I'd start with RAID 1, not 5. 5 will require a rebuild with a new drive if something goes wrong, while RAID 1 will work with 2 drives and give you complete mirroring. Since you intend to have a "local" backup copy anyway, why not just skip that and use RAID 1? It's literally the same thing, except it'll actually provide uptime in case of failure, unlike a backup drive or raid 5.

3. So RAID 5 plus a local backup, plus another offsite? This is overkill IMO. (Not the offsite backup that's good. But raid+local copy. Just use two drives and mirror them using whatever you prefer.) In your place, I think I'd go with BTRFS in raid1c2 mode. This is like raid1, in that with two drives, you only get the capacity of one drive. But, the "c2" means that each data block is mirrored to two drives. With more than two drives, you can expand storage. (With three 2TB drives you'd get 3TB) You don't get as much available storage as with raid5, but you get expandability, which you normally don't with raid1. And you get uptime in case of failure without an array rebuild (though for this you must mount the volume with the "degraded" option, unlike actual raid using mdadm). You also get filesystem snapshots.

4. You intend to do this manually? That is fine. My current solution is a second NAS at my dad's home, to which my system is backed up daily using Kopia. Kopia deduplicates and compresses the backups, efficiently keeping versions up to two years back. The simplest version of this would be a router that can host an FTP server using an external drive in its usb port. This way you could automate off-site backup and have it happen more frequently. Asus routers can do this, and even come with free dynamic DNS and automatic https with letsenrypt. You literally just plug it into WAN somewhere, and you'll be able to back up to it over the internet.

Finally, just some mentions.

MDADM, is what you'd use to create a software RAID array.

BTRFS has built-in multi-device storage, of which only single, raid0, and raid1 are stable. Do not use the raid5 and 6 modes. While named raid, the modes differ from actual raid. BTRFS is able to convert from one mode to another, and can add drives in any mode (though will need to "balance" the drives after changes, to make additional capacity available). It is also able to evict drives. It will not auto-mount a volume after drive failure, and requires the "degraded" option be added.

Mergerfs can be used to merge filesystems to expand storage non-destructively. It is able to arbitrarily combine volumes of any type, to combine their capacity. This way, it can for example be used to expand a raid1 array by combining it with a single disk, or another raid1 array, or whatever else. This can be done temporarily, as the combined volume can also be disassembled non-destructively, with each file simply remaining on whatever drive they were on.

I think that sounds pretty solid to me. Realistically you should count on having 3x drives for your important data:

1. The main data drive(s)
2. Drive(s) for redundancy, mirroring the data drives. (I use btrfs RAID1 for this.).
3. Offline local hard drive(s) that you keep somewhere relatively safe that you occasionally backup to.
4. (Optionally) Some kind of offsite backup.

So if you plan on having 2TB of data, you'll ideally want 3x 2TB drives. 2 in the PC mirroring eachother, and 1 in a closet or safe that you plug in and backup to a few times per year. (With bonus points if you can get another 2TB of off site or cloud storage to also backup to, in case of catastrophy.)

As for how you build it, I think it doesn't matter too much. Its possible to use whatever random spare PC parts you have to make a decent home server, imo. A lot of people on YouTube and Reddit have all kinds of fancy servers in a rack, but an old repurposed desktop can be fine. ( I would probably use new, decent quality drives though.)

Others have answered most questions but I just wanted to point out a few things:

NAS is network attached storage: a separate server which makes 'shares'/volumes available over the local network. A DAS is 'direct attached storage' which plugs directly into a machine. Since you have a server, a NAS is probably the right route. https://www.techradar.com/features/das-vs-nas-what-is-the-difference

Good that you mentioned both backups and redundancy as they're not the same. https://www.tencentcloud.com/techpedia/108368

Some people want a dedicated machine for being a NAS, while others want to make use of the hardware by making it pull double-duty as a server. I have an old PC I loaded up with drives and installed truenas on: I made zfs pools (opposed to raid) and exposed shares to the network. I can set up virtual machines or use "plugins" / jails for hosting other services like immich etc. E.g. https://www.truenas.com/docs/solutions/integrations/nextcloud/

Regarding backup versioning, modern filesystems like zfs and brfs have snapshot features. Regarding "one backup", it depends how important it is to you. https://www.backblaze.com/blog/the-3-2-1-backup-strategy/

Depending on the board in your mini-server, you may have enough SATA ports to plug in directly. I have a system similar to what you're describing (N100 with 4x 2TB HDDs with 1.5TB data): 2 of those drives are set up in RAID1 (mirror), and once a month, I plug in one of the spares, rsync the array to it, and unplug it. Every 3 months or so, I swap the offline drive with an offsite drive. I used to use a USB dock for the offline drive, but I got a 3-bay hot-swap enclosure to make the whole process faster and easier.

The server shares the array via NFS and SMB, and it is absolutely a NAS for all my other systems.

If you expect to exceed 2TB data within 2 years, then you'll need to replace all 4 of those 2TB drives in 2 years. You might, today, get a pair of 4 TB drives and one 2TB, use the 4TB as your main storage, the 2TBs as rotating backups, and wait until you actually outgrow 2TB to upgrade the backups.