Is it your island on Animal crossing? My daughter had a space on my island prior to getting animal crossing. When she got her own copy she started her own file on her own island without problems. The only sacrifice was not being able to download QR codes, as she had done that on my island. However, this is where things get complicated. She used to have an account on my Switch. When she got her own we had problems transferring her profile over. Can't remember all the vagaries but it ended up that we somehow "duplicated" her profile over the 2 switches. Weird but we learnt how to live with it. Her having her own island in ac on her own switch was, apart from inability to transfer QR codes, plain sailing. Not sure if this actually helps you, as i feel my situation is pretty unique.
I've never even played it. She's got her own profile and her own save data.
I knew be Splatoon 2 was in the same boat but I watched a video a week ago and the person said they transferred it with no problem. And you know what? I suddenly wish I had ordered the regular Switch and not the lite. I could have kept the extra dock. And she's gonna have to play the game on my Switch, anyway. I don't think it'll hold up on the Lite.