So I looked into game dumping law

It seems DLing copies of software you own and ripping it yourself are both legal grey areas because they are without precedent and subject to what counts as "fair use".

DLing is the darker grey as it's usually seen as a direct copyright infringement bc the shared copy is unauthorised (I couldn't find the relevant laws). But it's arguable on fair use since there's no officially sanctioned way to play the game on an emulator (which is legal by precedent).

playing your own backup *relies* on fair use (US Code 17 § 107, which e.g. explicitly mentions for "research"), hence on a subjective matter with no precedent. W/o fair use, US Code 17 § 106 says Nintendo has exclusive authorisation rights except for the 2 cases mentioned in US Code 17 § 117. I can't find the WW EULA but the Switch one bans all copying of Switch software. The exceptions are:
– the copy being required to use the game, or
– that it's for archival (= backup) purpose only,
neither of which applies here. (Notice that homebrew often uses the term "backup" to appear legit in that sense). There are also additional laws (part of DMCA) I cba to check apply here that forbid circumventing copy protection, i.e. using homebrew to make the copy.

Two more points:
(a) this is USA law only. I don't wanna do more research but I'm pretty sure making a copy of any copyrighted medium is completely illegal in the UK. It was legal for a bit during 2014–15 but got quashed and they gave up.
(b) de facto is important – nobody's been sued for dumping a copy and nobody's been sued for downloading a copy.

some sources (all interpreted from lawyers but still contradict each other a bit):
https://www.law.cornell.edu/uscode/text/17/chapter-1 – primary source
https://www.tomshardware.com/uk/news/why-most-roms-are-illegal,37512.html – secondary source
https://www.howtogeek.com/262758/is-downloading-retro-video-game-roms-ever-legal – secondary source
https://www.copyright.gov/help/faq/faq-digital.html – secondary source