I have never seen someone so confidently incorrect about their setup.
Your SD cards will be completely burnt up in a few months. Use an external hard drive! You can zip tie the power wire to the Wii U's power supply or something to hide it! The USB cables are actually quite small as well. They sell them in a lot of different colors, including white and black. You can also get a longer USB cable and hide the hard drive in a cabinet or on a shelf behind your table or something! I have a fully functional USB 3.0 hard drive cable that is 3 ft long and it works just as good as a short 3-in cable
Don't use USB flash drives, they burn up within days if you use them for stuff like this. It's like defragmenting an old solid state drive. If that last sentence doesn't make you cringe I don't know how to explain it to you better.
The maximum theoretical size of a master boot record partition is about 2.2tb. Wiiflow and usblodergx technically support GPT, whose maximum partition size is several exabytes!
The theoretical limit for FAT32 is around 12 TB, so that's your limit on Wii drive size I guess. Not sure how you would fill it though since the entire Wii+GameCube library is only around 7tb.
Anyone still using the GX emulators... I don't know what to say, honestly. That's like using ie6 on a windows 11 machine. Please read the tutorial on RetroArch, at least! It's actually pretty easy once you know the basics of what you're doing.
Let's talk about your use of d2xcios beta 53. NO! STOP! Bigger nomber doesn't mean newer or better! Please go grab the newest version of the installer from
https://github.com/davebaol/d2x-cios and use beta52 in the installer. beta53 is buggy (dozens of games just won't launch because of bugs it introduces), outdated (it's an older version), and has never been officially recommended by anyone except those YouTube tutorial videos that try to get you to brick yourself.