One problem I've found with many filter systems when using high density filters for long exposures is light leaks. That can be a particular problem with newer filters that have oleophobic/hydrophobic coatings. The adhesive used on light sealing gaskets won't stick to the filter so the gasket has to be on the mount instead. To avoid scratching the gasket cannot press onto the filter, there always has to be a very small gap.
The problem with the GF 23mm and Lee standard wide angle adapter is the groove at the back of the adapter into which the lens barrel fits isn't quite large enough and doesn't allow the adapter thread to engage completely. Lee have an adapter specifically engineered for the 23mm which doesn't have the problem..
If you don't want to use ND grads then the Kase magnetic system of circular filters may be useful. I've started to use it with the GFX50R. It's particularly good with high density NDs, no light leaks with the 23mm lens and 82mm system. You can stack 82mm filters with a minor reservation. 23mm with 2 filters, or polariser and 1 filter, are OK, polariser with 2 filters slight vignetting. Kase now have a circular ND grad system where the grad can be slid to move the transition point but I suspect the amount of movement is limited.
The main parameters that determine if a filter will vignette are lens diagonal angle of view, filter aperture/diameter and the filter distance from the lens entrance pupil. Because the entrance pupil of wide angle zooms can move a lot filters need to be carefully checked.
I've used the H&Y 100x150mm filter system with drop in ND/Polariser, Polariser and NDs, very occasionally had light leaks with long exposures. These were mostly due to the circular drop in NDs being slightly loose in the holder which could be permanently tightened by squeezing. The H&Y filters are excellent but tend to be heavy with a full pouch. That's why I'm trying the Kase magnetic circular system with no grads, total system is very light weight and compact.
Bob.