Have the Steam version and have the update paused. Did nvidia kill SLI support for camera support as the do in every "Upgrade"? Have 2 1080tis and game played fine in 4K. But I didn't get very far in, so lost saves aren't an issue. Remember watching a YT video of The Witcher 3 running butter smoth in 4k, outperforming a single 2080ti.
I have two GTX 1080Ti's, and before that two GTX 670's, and The Witcher 3 used to run fine with SLI years ago. I downloaded the new, upgraded version to see how the game had been improved and to maybe start playing it again (like you, I didn't get very far originally). SLI is totally broken now. The game runs but the graphics are corrupted. I have tried several versions up to the latest (4.03) and they're all unplayable with SLI enabled on my i7-6900K Windows 10 machine. I tried the DX12 version and that looks OK but that's only because it doesn't use SLI at all.
Don't buy the new GOG version hoping that SLI works. You can opt to download the classic version, though, and I expect it still does SLI.