the benefits of a nas is a few fold.
1. a box to put in extra hdds
2. it can automate proceses for your hdd maintenance/monitoring such as, scheduled smart tets (short/long) and also monthly raid scrubs)
3. networking capability. The NAS and it's shares can be accessible very easily on the Lan, which is probably one of the core reasons for getting a nas. This is unlike external usb storage devices where you have plug in directly to access and use. All NAS cares about is that it's on the same lan.
4. you can use a NAS for remote access. However i only suggest doing so via VPN server setup your router, and have client devices access via a vpn client. You can also do plex streaming remote for contents hosted on the nas (according to others, this seems safe to).
5. Quts hero with zfs can go further using their raid z1 and z2 solutions to help alert you when your data becomes corrupted. If you simply store data on a single HDD, there are NO indication that the data stored on it went bad. You just one day access it then noticed music has weird noise artficacts, or pictures having visible artifacts in them. ZFS aka quts hero protects from this gradual corruption of data. This is why i prefer storing my data on a nas rather just a non raid solution.
if you don't need many of the things i highlighted above, then usually an external usb storage like a WD elements or seagate expansion should be able to handle your backup needs.
Just remember though that a backup is having the SAME duplicate of data stored on DIFFERENT storage devices. So if one goes back, you refer to the other storage device.
Also why still using CAT5e? switch to cat 6 or higher. the prices are quite affordable these days. However even if you do upgrade cables this does not mean you will jump from 1gbe to 10gbe. For that, you would need to also upgrade your switches, and also devices with 10gbe port capabilities on BOTH ends e.g. from device A to device B (both must have 10gbe, ontop of the switch between having that also. ONTOP of the ethernet cables also supporting 10gbe capability). Only reason to get better than cat5e is just for future proofing.
I recently had a faulty cable, so i switched that out for a cat7, since it was cheap and 10gbe ready if later i do get switches to support that. Anyway i managed to fix my problem and the switch reported a steady and stable 1gbe wired connection when tested from 1 room to another
also in regards to ethernet cables get reputable brands. i use the local online retailer so i also check the reviews to see what issues others reported with the cables BEFORE buying.