Depends on how much you want to invest for the camera.
Personally, the cheapest option, which would give you the best in quality of footage is probably the Blackmagic Pocket camera: it allows you to shoot RAW, Pro Res,10bit,…, sure you’d learn so much in term of color grading, you could pair the camera with a metabones speedboster to reduce the 16mm crop factor and gain 1 stop. (It doesn’t have slow motion though, and it doens’t record more than 1080p). I think the Blackmagic micro cinema actually records slow motion, and it has all the features than the pocket has.
If you searching for something to shoot slow motion with (good camera also for photography), I personally really like the Sony a6500 (yet it records 8bit 4:2:0), and it's cheaper than the A7s II.
I haven’t tried the GH5, but never really liked the GH4. I don’t know, there’s something about that camera which I never really liked (even if people keep saying it’s got lots of better features than the sony a6500), but again this is just personal taste.
Sony FS7 is great, but way more expensive. As Robin said, you could find a used one, which could be quit cheaper!