So you want something like the Sky3ds or 360 DVD stuff where only 1:1 copies of original games in region if that is a thing for the device in question get loaded (no
cheats, ROM hacks, region free, downloadable games...) but presumably working on an otherwise stock device? Or would you settle for modifying the firmware as well such that the cart could function as basically just another storage method like the USB and SD methods available already (but having whatever the current requirements are to get a custom firmware going on)?
Personally I would develop the latter first and then see about figuring out if you could fake whatever full set of read protocols, encryption and additional data (don't know if Switch games are like the 3ds and some DS stuff in having a unique header/signature somewhere in it all that could be used for authentication/duplicate detection) that might be needed, and implementing a means of switching as you would presumably not have a software means of doing it*.
*technically the 360 drive emulators also presented a USB full of pictures that it controlled, you select the picture of the game you want to play with the 360's in menu picture viewer and the drive emulator dutifully loads the disc image. Whether you could or would want to do that here I don't know.
Afraid I don't have any real technical info on the Switch like I might have for now older devices. I would probably look first at whatever open source custom firmwares have in this regard (though likely minimal as most other checks probably happen in the menu)
https://switchbrew.org/wiki/Lotus3 and
https://switchbrew.org/wiki/Gamecard appear to have a few things, though probably not enough to even attempt the modded firmware approach noted above.