S5 is definitely "better" image quality than GH5 imo, and that certainly factors into my choices, but technology is at a point where pretty much everything looks pretty good. So if all my lenses were MFT, or if GH5 had some other feature that I really needed, I'd be happy with GH5 as well. Better photographers than I might have a more nuanced look at photo ergonomics, but to me all hybrids are roughly the same amount of pain when rigged for a narrative shoot.
For my amateur photography, every camera I touch reminds me that the NX1 was more comfortable 😞
The S5 has a great APS-C crop mode, which will pair nicely with your Sigma 18-35. You're already in the system, no pressure at all!