Ok friend,
I've tried a few experiments and I can tell you this:
- it's not a ìn SD issue, even with no SD in, the original cartridge of BOTW (a working one, run smooth on a second Nintendo Switch) crashes after some time
- it's not an overheating problem, I played the BOTW cartridge for about 15 min, then put the switch in sleep mode for a whole day to let it cool down, and then - after 5 or 6 min - the game closed because an error occured (the switch was cold as hell). Same error of the NSP on the emunand and same behaviour of the Switch afterward.