I played The Witcher 3 Wild Hunt when it was new: I played Wild Hunt when it was new, but not the Hearts of Stone or Blood and Wine expansions. Then other things came up and I kept putting it off, but finally there was no realistic possibility of actually playing it, "I'd like to complete the experience on this platform, even if it's while riding the commuter train, but I'm not really ready to play the base main game again. If you are obviously in a very particular situation, the latest "Switcher" update will satisfy you greatly.

The Witcher 3 for Switch is the Complete Edition, which includes the story expansion and "all downloadable content released for the game...As of the 3.6 update, it is also compatible with existing save data on Steam and GOG." You can continue your PC adventures on the go, and the HUD, menus, and Gwent touch controls are also supported."

The Switch version of The Witcher 3 includes a "cloud save" menu, which, as Kotaku explains, allows you to log into the appropriate account and select the saves you want to roll over. Be aware, however, that "mod-related bugs and blockers" can be brought in with the save and cause headaches, and that if for some reason you change the filename of a save, the Switch's cloud save feature will not be able to find that save.

It also appears that another method is possible, as the menu provides an option to upload local Switch saves to the Steam cloud. We contacted CD Projekt to clarify whether this means that you can actually go back and forth between Steam and Switch, for example, if you want to play on your Switch while on the bus to work and then continue on your PC at home. (I can't imagine what the point would be otherwise.) In the meantime, it might be a good idea to back up your local saves before you start tinkering with them so you don't accidentally overwrite old saves.

Update: CD Projekt has confirmed that cross-platform saves work in both directions: you can go back and forth between the two platforms as you please and take your game with you.