I still love the image from this camera. It looks amazing.
Due to the engineering of the most lenses, they are usually sharpest between F/5.6 and F/8.0. Stopping down lower than F/8.0 will usually increase softness and stopping higher than F/5.6 will usually increase softness. (It's a good rule of thumb, as very lens will always be sharp within the F/5.6 and F/8.0 "sweet spot.")
DXO Mark is a website that scientifically analyzes lenses. It is a great reference tool for individual lenses and how they are engineered.
Also, according to DXOmark, the Sigma Art 18-35mm F1.8 is actually quite a lot sharper than the Canon 50mm F1.8 prime, so unless your lens is defective, than it is not the problem.
Between those two cameras, (despite the budget) I would still choose the Canon C100.
If the GH5 sensor is anywhere similar to the GH4, the dynamic range is heavily tilted in the direction of highlights rather than shadows. This makes it an amazing camera to shoot outdoors with, but it struggles indoors with shadow detail. The C100's 12-stop dynamic range is well-rounded in the middle of both highlights and shadows. It can handle them both.
It's higher ISO capabilities of the C100 can come in very handy as well. I don't think you'll be disappointed with this camera.
Definitely the 12-35mm f2.8
The Mitakon Speedmaster 25mm f0.95 is both a whole lot cheaper (found for $369 on Adorama) and with tested better image quality than the Voigtlander alternative, and for a Micro Four Thirds lens with a brilliantly fast aperture, it's (in my opinion) the best choice, but I find myself still using the Panasonic 12-35mm f2.8 OIS lens more often. It's just too versatile.
In my opinion, the absolute best camera you can get in within that price range is the Panasonic DMC G7. It's like a Panasonic GH4 without some of the extra features (such as slo-mo video) but it shoots a brilliant and cinematic 4K video image and captures great still photos as well.
Here is an example of it's video footage:https://www.youtube.com/watch?v=FUxDvGD2Ask
You can find it on eBay for $436, but you will also need a lens. If you insist on a very inexpensive zoom lens, you can find the Olympus M. Zuiko ED 14-42 f3.5 - 5.6 Micro Four Thirds zoom lens (currently found on eBay for as low as 89 dollars.)
The total cost could be slightly over $525, but well worth the extra money as nothing else in that price range can capture image quality this good. The Panasonic DMC G7 is also an interchangeable lens camera, for more versatility in the future if you decide to invest in different lenses. With an inexpensive lens adapter, it can also take Canon and Nikon lenses.