https://www.speedrun.com/smo/thread/f89kk

You could hack your switch and game dump super mario odyssey, but it's really complicated.

I'm pretty sure as long as you have a Nintendo Account, it still keeps all your games. I don't know about Save Data, I recommend doing more research before you do it.

Factory Reset depletes all your game data regardless, either that or switch homebrew which people use to downgrade now without factory reseting if they wish, join the discord and ask there if you want to know more about it. We'll be getting Cloud Save in early September so it will be a safer way to keep all of your game data while downgrading/deleting game files and such, until then, these are your 2 options.