Obviously finaldestination is empty to start with so it could just be a direct copy of your first hdd - so make that the largest drive.
(I'm saying copy here, presuming you want to keep the old drives for now, just in case you accidentally delete the wrong stuff on the 4TB drive)
Maybe clean up any obvious stuff
Remove that first drive
Mount the next and copy the data to /mnt/4TB/source2
Now use rmlint or fdupes and do a dry-run between source2 and finaldestination and get a feel whether they're similar or not, so then you'll know whether to just move it all to finaldestination or maybe then use the gui tools.
You might completely empty /mnt4TB/source2, or it might still have something in, depends on how you feel it's going.
Repeat for the rest, working on smaller & smaller drives, comparing with the finaldestination first and then moving the data.
Slow? Yep. Satisfying that you know there's only 1 version there? Yep.
Ongoing support is a separate discussion... could be money, could be beer... but I agree, it should be discussed so that a dependency is understood / avoided
My choice is Arch Linux purely because it's bleeding edge
I've no idea if Arch actually has newer drivers than Debian / Fedora, but if they are you'll (usually) get better support from the developers of whatever application / package - or in your case - drivers that you're facing.
It's more involved than "just" installing Debian, etc... but reading through the Arch Linux wiki as you install will (should) ensure you've got the correct drivers setup and you'll know why they're working.
So... it'll be more effort, but you might get "better" results.
Fail2Ban will block login attempts (ie from anywhere)
I have a similar setup with pfSense, pfBlockerNG, HAProxy, etc, but I keep F2B running on my DMZ server in case something is ever compromised as it'll block / slow down anyone trying to move around the network.
Ah, there's other ways too... see the video with a drone watching HDD LEDs.
Needs malware to do the LED thing of course, oh, and deaf security that aren't concerned by a drone hovering outside a window every night, but apart from that, it's foolproof.
Boot memtest86+ and let it run (overnight...?) that's the simplest & easiest test - even if the RAM is ok, it might show other problems (over heating, etc)
This has been the case for years. That's why there's international standards to defend CNI systems... not sure this is new news