Okay, thanks
.
After a quick search, I found a whole 2 posts with a similar issue over ~10 years.
To be clear, after a *cold boot*, straight into Nickel, what happens (in detail) if you try to enable WiFi *from the Settings menu* (and not the WiFi top icon)? (Since I assume a Kobo boots with WiFi on, you may have to kill it first, but the results from the same experiment in the default boot state are helpful, too).
As @davidfor mentioned, in the few posts this was mentioned, there was talk of wiping the wpa_supplicant config. In my experience, Nickel's never had any trouble doing that on its own when it's having quirky WiFi issues, but, YMMV, and WiFi is finicky on those devices, on a whole lot of levels, so the job's not necessarily as straightforward as one would assume.
I did not manage to find a KoboRoot to do that, though, so, here's one, that'll replace the config with the default blank template.