Thanks again for all your help.
I've encountered a new problem...
The USB install has worked, as far as I can tell.
However, ACNH keeps saying "corrupted data found."
Reinstall, same issue.
Doing some research, I understand there could be two problems (amongst others).
1. Firmware issues. Not 100% sure how to check what firmware Atmosphere is running.
2. Sigpatches. I'm reading
https://gbatemp.net/threads/how-to-create-your-own-sigpatches.616288/ and trying to understand what's going on, but the lingo is very new to me.
Are those the likely culprits?