Okay, so... since nobody seems to want to make a jailbreak for a Mariko Switch, I eventually found someone among my network of friends that was willing to sell theirs... it was in the "potentially patched" range, but evidently hacked. Got it with some dongle called an RCMloader, and the rest of the stuff it came with stock.
At least after figuring out how to make a partition for emuMMC on the SD card, I was then able to finally get Atmosphere to work.
My previous Mariko switch, which has all my legit save files and stuff, is still on 8.0.1, as it shipped in the box.
This new-to-me unit is on 12.1.0...
Simply, I need to find a way to migrate my present user profile to this one, so I can give the Mariko unit to the wife because I know she's been itching to play Metroid Dread since launch... and I think she managed to find a place to score the Special edition copy. I don't know how to do that, and to be fair I do feel lost in terms of what all there is to do with the system, and wondering if it's okay to delete the former owner's user account and keep the presently installed stuff?
This is my first time messing with this stuff on the switch, and well, didn't expect to have a pre-hacked unit to work with. So I'm still kind of in the dark for how to proceed.