Further update on Project Murphy: I fucked up.
I broke the battery connector off the logic board. You know, the thing all the tutorials tell you to try very very hard to not do. The thing I had managed not to do during fuck knows how many times I'd connected/disconnected batteries from the damn thing trying to get it working. (Which, to be fair, is probably partly how it broke. I really expected it to happen sooner, it's my first try, and I've had to keep redoing things on that board.)
I think the reason I feel so salty about it is that I was so close to the finish line. Long story short: it wasn't the SSD that was the problem, it was Rockbox. I encountered multiple kernel panics after I'd installed the iFlash Quad, and it happened during playback of different file formats, and its own built in games. Those kernel panics led to one of the SD cards in the iFlash Quad getting corrupted, so I had to open the iPod up again to reformat it. The battery connector break happened as I was putting it back together for what I hoped would have been the last time, since I'd decided not to use Rockbox going forwards after all.
So yeah. It was galling, and disappointing. I had high hopes for Rockbox. What especially appealed to me was the simple drag-and-drop file management. But that one perk is not enough to keep me using it after the number of issues I've already experienced with it. So, iTunes it is, at least until such a time as they stop supporting it (although I'd imagine people would find a way to recreate it/keep it going, in that instance.) It has to be kept anyway, Rockbox or not, in order to restore iPods that have the sads.
Anyway - I await the arrival of another broken iPod, replacement facing, and a practise soldering kit. Because I'm not giving up on Murphy quite yet. I'm going to learn how to solder, and then give him a replacement battery connector, then see if he still works. (I don't know whether the logic board is cooked after what happened, which is why there's another broken iPod on its way to me as backup.) If he's dead after that, then I accept his fate and thank him humbly for his service. But let's see if we can't still get him back >:) ❊