I'm a big believer in the Switch being more capable than a lot of people think, but I just can't see it possibly handling a realtime version of Witcher 3, a demanding 8th gen open world game that can't hold 30fps even on PS4. This is like expecting the Vita to run Red Dead Redemption 1.
It isn't quite that extreme, but it would be a massive undertaking. It would require a lot more work than what Panic Button and the like usually do. It would need actual asset rework going beyond texture resolution. If you look at Dragon Quest XI S, you can see kind of an example of what would be needed. The Switch version features a lot of newly created versions of various assets. Of course, that doesn't address the CPU issues, which is the real challenge for an open world in the style of Witcher 3. Which is where the game might be able to juuuuust about almost get by on the GPD Win 2 despite the glaring GPU deficiencies vs the Switch.