On reading your GH4 list of 'issues' - I feel your pain.
I'm really not happy with the colour rendition from mine, even with MWB. There must be something that can be fixed using a LUT / standard preset to get rid of the Purple tint.
(Alas all the playing in FCPX using 3rd party tools I can't figure it out)
Isn't there a 140mm dome for the 8mm - would that help with the spilt shots?
I think, to be honest swapping out to another system won't solve your issues, yes the ones listed, but then you will end up with a another list for the camera you choose.
From a rumour going around, the GH5 prototype is currently in the same body as the GH4 - so might be worth waiting till there is news of that - as if so, some things (ISO, color etc) might just be a camera upgrade away.
Another thing, which you haven't mentioned, which is another annoyance for me is lack of IS on the 7-14mm - using a FF camera with those not so wide lenses will have IS.