questions regarding this sale as well as cross saving

Hello, I'm currently looking into buying the Witcher trilogy because I have heard incredible things, especially regarding The Witcher 3. I have two main questions, though

1. I also want to get The Witcher 3 on my Switch 2. I heard there is cross-sav,e so having that option would be incredible. I was told that I have to get the complete edition for both systems so that the cross-save would work properly. Is this true?
2. That leads me to my follow-up question. On Steam, there is TW3 for 10$ and the trilogy for $11. Could I get the trilogy bundle, which includes the Witcher 3 base game, and later on buy the DLC, where it would serve as a "complete edition", and my cross saves to the Nintendo Switch work fine? Or should I get the Witcher 3 complete edition and then buy the first two games individually?

Thank you for the help in advance 🙂