I have an S1, and had an S1H for a month. Haven't used the Nikon or Fuji so I won't comment too much on those.
The IBIS is great, though panning slowly can cause it to fidget a bit. I handheld shots using the 70-200mm f/4.0 and the 2X teleconverter attached and I'm amazed. It's almost as good as the Olympus IBIS.
The low light is stupendous. I used a 35mm f/1.2 Sigma Art lens on both S1 and S1H and I couldn't believe how much I could get away with while using it in the dark. I almost bought a Z6 myself, but opted for the S1 because I read that the Z6 can cause banding in low light situations due to auto-focus phase detect pixels on the sensor. I guess you really need to push things to make that patterning appear, but I decided to avoid the issue entirely. Of course, now I'm stuck with Panasonic auto-focus haha, which honestly isn't that bad for me at least.
The audio is OK. It has mic and headphone jacks. Mic levels are easy to see. You can set some of the buttons to help you deal with audio levels. There's a plug-in power Mic option as well as Mic and Line recording. I'd consider the Panasonic XLR hotshoe adapter, however, if you want to improve on that.
As for photography, I found both S1 and S1H really great for shooting stills. There's not much difference in image quality, except the S1H has the anti-aliasing filter on it. Shooting stills, you would have to pixel peep to notice it.
There are some great lenses available now for the S1 L Mount. I've personally used/own:
-Lumix S 24-105mm f/4.0 (kit lens)
-Sigma Art 35mm f/1.2 DG DN
-Lumix S Pro 70-200mm f/4.0 with 2X teleconverter
-Sigma Art 14-24mm f/2.8 DG DN.
The 24-105mm wasn't my favorite lens to begin with, but it has really grown on me and I find it to be a versatile all arounder for stills and video.
When it comes to price, you pay more for the S1, but it's quite the camera, and I feel it's worth it. I also have a GH5S, and I barely use that anymore now that I have the S1. It does everything, and the only thing I can think that would improve on it is the S1H, which has a load of awesome features and it's so easy to use without the need for a pile of kit bolted onto it. Same goes for the S1. You don't really need anything extra to make this camera work for you while shooting stills or video. This is my experience with it, you may have more requirements.
Good luck with whatever system you choose, and I hope this was a bit helpful.