I looked for that on the American Amazon site, and since the 5 TB model isn't listed I decided to go for the 8 TB model. I'll try to recreate your setup with that larger drive. I know it's overkill, but 8 TB isn't much more than 6 TB and if the Wii U scene ever supports loading WiiU backups from the same larger drive as the Wii and GC games it would be worth having. and if it doesn't work, that 8 TB drive can instead go into the RAID 6 network drive I'm putting together.
https://www.amazon.com/gp/product/B01LQQHLGC/ref=ox_sc_act_title_1?smid=A2YR5Z6BYZME44&psc=1
and I was able to find the same 2 TB drive here:
https://www.amazon.com/dp/B06W55K9N6/ref=psdc_595048_t1_B07DXLSMBC
The USB hub I have been using for my two Switch consoles is this one:
https://www.amazon.com/gp/product/B01FO2QS0O/ref=ppx_yo_dt_b_asin_title_o01__o00_s01?ie=UTF8&psc=1
Which works great for the Switch at least, but haven't tested it in the Wii U yet. (for that matter, I still need to get around to actually finishing the soft mods for my Wii U) For the Gamecube adapter, I ordered this to see if it works:
https://www.amazon.com/gp/product/B00BD8I3EI/ref=ppx_yo_dt_b_asin_title_o01__o00_s00?ie=UTF8&psc=1
This idea is that I would plug the gamecube adapter into it, and then it into each Nintendo Switch and the Wii U. I can then push a button to alternate between the active console, rather than having to either get multiple sets or move things around.
...
It will take me a while to get to all of this because I am in an active semester while working on a console project, but I'll report back how it goes. The project I am working on is nearly done: I designed and built a 6 foot wide piece of furniture on wheels to house all of my consoles, but it's more than that: It's wired. The underside closes to hide all the major wiring, which includes surge protection and voltage regulation, a 24-port TP-Link gigabit switch for all the consoles to be connected to, and on the back I have 6 Cat-6 ethernet ports - one of which is for the WAN to plug in, and the others for possible LAN parties to hook up. Everything setup to use HDMI, with two 8-port HDMI switches (so a maximum of 14 devices) and all wired to use 4K HD, with cable lengths set to ensure that I can use 4K at any time. In the back as well it has 2 HDMI ports to plug into either one TV, or two different ones. I also set it up to have 6 integrated electrical ports on the back. On the front side I integrated a USB charging station - 13 ports at 2.4 amps, 1 fast charging port, 1 USB-C charging port... so, in the end, my consoles will all be in one place, with plenty of room for hard drives behind them, no need to mess with power cables or wiring, easy hookups, and I can move it by unplugging this one piece of furniture from the wall and disconnecting it's Ethernet cable, and can move it anywhere.
Actually, it was originally designed to also house a cyberpower UPS for an hour of electricity in the event of a power outage or if I just needed to move it, but it didn't like being contained like that (it's fans are really loud when it needs to use them) so had to offload it into the basement, but the voltage regulator fits nicely and it can still be plugged into the UPS - just not mobile with it. I also l need to integrate USB powered fans for the underside of each console since the wood is so thick as to be insulating and need to direct airflow out.
The goal is to be able to play all of my consoles without having to pick and choose which is plugged in where with messes of wiring. Although, that would be easier if I could find a USB switch that works the other way: I plug it into the Wii U, I plug more than one hard drive into that, and then I switch between the drives with a button press.