I owned the sony 50 1.4, but I found the resolution and contrast poor when used between f1.4-2. After that, it's great.
I considered the Sigma 50 (I also owned a Sigma 85 1.4), but I am migrating away from a-mount. I don't like Sony's lack of support for serious shooters. So, with the Nikon 50 1.2, I can use it on mirrorless systems (aperture ring) or toss it on a nikon body. That's why I had hoped for a simple converter at first. I had just hoped to mount it on my a850 while I stil have it.
If you feel Nikon is more you style, then indeed jump system. I would advise to go ahead, sell all your Sony gear and buy all the new Nikon you'll need and don't look back.
I know it feels tempting to try to build bridge between the systems, but it's not going to work satisfactory anyway.
So, either stay with Sony or go all in with a total system replacement - yes it will cost you, but in the end you will be happier with a system more to your liking.