About the GH6, I've not used it a lot but even with HLG and V-Log (H265) and I feel the details rendering is slightly different compared to the GH5, maybe because of some details filtering ("Intelligent detail filtering" from Panasonic) but there is a difference, like if the GH6 smooths and sharpens some details in the same time, a bit like with the S5II standard profiles but far less pronounced.
https://www.youtube.com/watch?v=55RygnQqAbU
I think Deezid used the GH6 a lot more than me thought and said Prores has less noise reduction so maybe the details rendering is better than with H265.
These days, many brands use a lot sharpening and different type of noise reduction, sadly most of the time there is no way to disable them.
It is true that there is no chance your clients could spot the loss of details, but I use Panasonic cameras since a very long time and I loved the way I could crop or extract picture from my videos while retaining a lot of details, like a true Jpeg picture without too much noise processing or digital sharpening. We can't do this anymore with the S5II standard profiles because there are too much processing going on internally, exactly like on smartphones.