My Tokina 11-16mm works great on the URSA Mini 4.6K as a wide lens. I'd suggest looking into that as an option. There is the photo version and cine version of the lens. The photo version is F2.8, while the cine is T3.0 if I remember correctly.
Additional note: I checked PCAM on my phone. The GH4 would need a 7.5mm lens to match the URSA Mini 4.6K with a 11mm lens. So if the GH5 has the same sensor size as the GH4 then you should be in the ballpark with the 11-16 Tokina for what you're looking for.