I don't understand your problem.
In menu item "gear/D2", there is a setting "Live View Boost". When set "OFF", the EVF follows the exposure from under to over, etc.
If my subject is under or over exposed, I adjust the exposure properly.
When shooting in a studio using flash and manual exposure, the EVF is dark before the flash fires. So I choose to set "Live View Boost" "ON", in order to be able to see and compose my image.
The mileage of a Panasonic user may vary.