Looking for the simplest solution for local backup setup as step one of my eventual 3-2-1 backup goal.

Precursor:

Unraid is my first real NAS aside from a 3 generation Windows PC setup running a continually growing "Stripe Raid" as my internal storage. Dumb, I know, it is why I knew a genuine NAS was inevitable. I always referred to as my HTPC. It housed my Plex server as well my ARR programs and NZBGet (now running SABnzb). Not only was it my "media server" but it was still just as one of my homes PC's on my network. All my important networks files between my Laptop and the HTPC would be backed up running a program called SyncBackPro (from the always on HTPC) once a week to a mirrored backup onto an external 5TB USB drive. Suffice to say, I not only outgrew that drive, but I knew the Windows setup needed a real NAS. So yes, I'm an Windows users of many years and consider myself an intermediate user, but I am still just an 'Interface/GUI/ guy. I knew that most NAS setups are Linux based and a learning curve will follow (and it sure has), so I chose Unraid for it's user simplicity for n00bs like me and great support. With that said, thank you for that and thank you in advance for any help you can lend on this post. With that said, my Unraid is running the latest build 7.1.4 (as of this post). All my ARR's are running smooth as well as Plex. But this is the tip of the iceberg, I want to really start using my home server for hosting more than just my Plex. I fully want my own cloud storage for one, but I am sure more will come as time progresses.

My Question:

If I understand correctly in simplest terms, a 3-2-1 backup is the best practice for file protection redundancy. And if I also understand, my Unraid Parity inside my Array is my "1", correct? If so then I assume my '2" can be a set up to a (just bought today) 18TB Seagate USB drive. Great, but I have one cavoite; I still want my networked Window's PC's to be able to back up to it as well, To do that, the plan was to partition a small portion of it for just that, and the remaining larger partition will be for my Unraid backup. I honestly have no idea, but can a partitioned drive have more than on file system?

So my question is:

As stated in my Precursor above, I am a GUI/Interface guy. I know rsync is widely used in Unraid (Linux) and I know a few Dockers work as a slick interface using it's commands. So that is what I would like to use. The two I am looking at are LuckyBackup and Duplicacy. Those 2 have great setup videos and writeups online and I feel they should work for my needs, but I am open to other options. I want to setup that aforementioned 18TB USB drive on my network. If Unraid runs a file system xsf (zfs for my cache), does it matter what file system my USB storage device runs if I did use it to back up my Windows as well (NTSF)? I am currently running a Netgear R7000 (Nighthawk) router running a custom DD-WRT firmware. As of now, aside from no 2.5/10Ghz/s speeds, it does fine for my needs. It has a USB-3.0 port on it and it can be setup to have it used as a SMB/NFS network backup storage.

So here's the kicker:

is that a wise choice to have my 18TB USB drive connected to my router to have as an accessible backup storage location for both my network PC's as well as my Unraid's backup? If so, can you point me in the best place to learn the best (easiest preferred) place to set that up?

For reference (is it matters), my current Unraid setup is as follows with my my specs listed in my sig: My NAS (Unraid build) is currently running a 8TB media storage dIsc, with a 8TB parity to complete my Array. I am using two 500GB NVM as my Cache, one recently added a protected drive per a SpaceInavders tutorial and is mirrored to the other. I fully intend to at least double my 8TB media storage to 16TB and for me that should easily suit my needs. Then, eventually when that money tree I planted starts growing, I will have a 2nd offsite backup to them make it Currently my "3" in my 3-2-1 backup setup. That is what a 3-2-1 backup essentially is, right?

Right now, I am just taking it one step at a time.

Thank you in advance for any assistance you can lend or point me to!

-TJ