TADataHoarder

What is the best method?

Sometimes the best method isn't a transfer at all. Don't forget transporting is an option.
Do you need to transfer the files? or could you just use an external you share between different PCs?
If an external works, but seems inconvenient, you can fix that by having the drive connected to a USB switch. When connected to a switch you can press a button or use a remote to change which PC the drive is connected to giving you instant access to the drive contents without having to sync/copy anything over.

Links below for examples of USB switches
https://www.amazon.com/Cable-Matters-Sharing-Computers-Peripherals/dp/B083JKDNRJ/
https://www.amazon.com/Sabrent-Computers-Peripherals-Indicators-USB-SW30/dp/B074TYDJK2/

fearing it may stop functioning in the future

It most certainly will.
If it has a removable battery, remove that ASAP, and it'll probably last a lot longer. If your phone isn't a total piece of shit then you can probably run it without the battery strictly from the charger's power. If you have a UPS, plug the phone's charger into that when accessing the phone. If the phone is a piece of shit and requires the battery to boot (some actually do, you'll have to check) you should consider buying a replacement battery or figuring out a way to wire DC power to the connectors if you have the skills for that. With a dud/worn out battery an otherwise perfectly okay phone may be stuck in a cycle of charging, booting, and depleting the battery during the boot process, and shutting off preventing you from accessing the device until you replace the battery or wire appropriate DC power to the battery connectors.

Is there a way to effectively preserve this phone, including all its data, in a virtual environment?

In short, no.
Android is designed in a way that "knows better" than you. You do not "need" to have low level access to the phone data, so Google/PhoneManufacturer has locked you out "for your own protection and security". With computers, it's pretty easy to clone a boot drive and boot it into a VM for archival purposes and the ability to migrate to different hardware in the future but phones actually go to great lengths to prevent this and that will be your biggest obstacle here.

If your phone was rooted and had the bootloader unlocked and a custom ROM installed you could potentially have an easy way to clone it all but that's usually never the case.

I'm looking for a method to create a virtual replica of the phone that retains all its contents.

This should really be quite simple but the manufacturers hate you. So, it isn't. This is sad, but this is just how it is. Preventing easy "stealth" cloning (cloning without unlocking store now/decrypt later style) is good but these devices should allow authorized cloning (boot, decrypt/unlock, then clone) to the owner but unfortunately they do not.

Keep the phone for as long as you can, but plan for it to die some day. If you cannot find a tool to back something up, set up a DSLR/mirrorless camera or suitable smartphone into a copy stand and record a video of the screen as you access whatever data may be in apps without a good way to export their data. It's a brute force method that will produce difficult to browse data, but it's foolproof. Screen recorders won't even work on Androids in all cases because certain apps block them from functioning. They claim this is for "your protection", but really it's at your inconvenience.

Expect 300MB/s transfer rate for the entire enclosure. If that's fine with you, it should be usable.
It doesn't support hotswap though. Removing a drive (or one erroring out) in operation will reboot (disconnect/reconnect) the entire unit power cycling each drive. Bad for a high availability system but fine for a cheap DAS for the price.

Uncorrectable sectors are bad.
Get a new drive, 500GB models are cheap/free. Buy three. Main+Backup+Backup minimum if you aren't already.

Try FreeFileSync for this.

It looks like all of your data fits on the 6TB and be backed up to your two 4TBs. Why aren't you doing that?
That would ensure an easy and simple way to maintain 3 copies.

4TB 2.5" drives and a big WD 8tb 3.5" drive that are old USB 2.0 externals in their factory enclosures.

I don't think any drive manufacturer sold 4TB or 8TB drives in USB 2.
Did you mean USB 3?

or do I need to buy an actual USB C drive like this?

USB-C is a connector type. It actually allows for USB 2 speeds. (30MB/s)
If you're going to buy anything get simple USB 3.0 adapters that support UASP. You'll get more than enough performance out of those.

Seems like a complete mess. Not sure what to tell you.
List the capacities for your drives and the total space used for each set of data.

How big is Data 001? Data 002? etc.
What is Drive A? What is Drive B? etc.

I can only connect 2 drives at the same time, maybe NAS could help?

Look for a USB DAS instead. Maybe a multi-bay docking station like a 5-bay that can let you plug everything in at once. Much simpler. A NAS would require reformatting your drives. Not bad for the future, just not ideal for right now IMO. Simplify how you access the drives you have in use before going to a NAS. They're much slower and usually limited by 1Gbps LAN.

At least you can actually order them on WD's website now.
$630 ain't a fun price though but being able to put the order in is something.
https://www.westerndigital.com/products/internal-drives/wd-gold-sata-hdd?sku=WD241KRYZ

You have to figure out how many SATA ports and drive bays each system has. A lot of Optiplex systems only have room for just 1 or 2 drives, sometimes just 1x2.5" and 1x3.5" with no room for two 3.5" drives which isn't exactly good for hoarding.

I'm guessing there's no SATA beneath all of this?

Correct, these are native USB drives.
You are stuck with this so you might as well put it back in the case.

Got my hands on my enclosures.
These Easystore PCBs can be placed into these MyBooks. They fit just fine.
The PCBs appear identical. The Easystore has an LED but that's just missing from the MyBook. Same board, different components, but most are the same. Even the winbond chip has identical markings but some others do not.