Ive moved over to Olympus with the Em1ii. I also have a Nikon D500.
For the past few months Ive locked the D500 away and made myself use the Olympus in order to get used to the system and eventually sell the Nikon gear.
For birds in flight there is little doubt that the D500 gave me more keepers. There has been much written about the focus tracking on the Em1ii, I think it would be fair to say that it isnt particularly good in comparison to the Nikon and I generally dont use it unless shooting against a clear sky.
Weight wise....the D500 and 80-400 that I have are definately heavyweight compared to Oly kit. However the Oly 300f4 is no lighweight either.
In fact the D500 and newer Nikkor 300 lens are lighter together than the Oly Em1ii and Oly 300 mm f4.
The Olympus IBIS is amazing and Sync IS (where the in body stabilisation works with that of the 300mm lens) is amazing too.
Image quality is of course slightly less with M43 as you would expect in comparison to full frame or APS-C, but not so much that it causes me any issues as I rarely print massive prints,
Overall Im pleased I made the move (though still cant bring myself to sell the D500)