You can download here the latest melonds core for Switch versión of Retroarch, haven't tried the switch version, but if it is the same as the android one, it should have Jit enabled + more settings available than the built in core updater:
https://git.libretro.com/libretro/melonDS/-/jobs/40613/artifacts/download?file_type=archive
Just download it, unzip it, and there you have the core, (well, there is a .elf and .nro)
https://git.libretro.com/libretro/melonDS/-/jobs/40613/artifacts/download?file_type=archive
Just download it, unzip it, and there you have the core, (well, there is a .elf and .nro)