I've made a new SysNAND backup and, knowing what I know now, I'm going to at least attempt the fastboot3ds route one more time.
Okay, don't you think you should be more suspicious of something you got cheaply using a coupon?
People get "brand-new" SD cards all the time that end up being of dubious quality. That's what h2testw is for.
You're not wrong, and I say this as someone that majorly dodged a bullet back 15 years ago when I actually "bought" what was probably a fake 1GB CF card on ebay, but the seller had their account terminated within 24 hours after I was supposed to (and didn't) send payment.
...but I'm unsure you're aware of who or what Microcenter actually is. This was an in-store coupon for a Microcenter-branded microSD card. I'm not sure there's a more reputable brick-and-mortor computer electronics store in the entire US. I mean, when I was there in February, they had Ryzen 5000 CPUs on the shelf, including the really in-demand SKUs like the 5900X,
at MSRP ($549.99) - that alone is a major sign of good reputation with the current ridiculous PC component pricing and demand.
Also to be more exact, the YouTubers in question were LinusTechTips and JayzTwoCents (though if Microcenter really wanted to up their sponsership reputation, they should sponsor Gamers Nexus). Here are two random videos of theirs that have the aforementioned Microcenter coupon sponser-spot (non-clickable links because I don't want GBATemp turning them into embeded videos and therefore junking up this thread with something that's kind of off-topic):
Linus: youtu.be/emlRd643NVI
Jay: youtu.be/K48-nO441pg
Or, if you
really want clickable links without dealing with GBATemp's auto-embedder, you can always use the following YouTube "proxy" (it doesn't proxy by default, but it can do so optionally):
Linus:
https://yewtu.be/watch?v=emlRd643NVI
Jay:
https://yewtu.be/watch?v=K48-nO441pg