I cannot answer all of your concerns, but I did switch from Nikon FF to the Fuji system a couple of years back. It was the XT-1 at the time for me, and I was able to use the Yongnuo 560 flash that I'd been using for the D800 without any hassle, with the exact same trigger system - cheapie trigger/receiver I got on Amazon. In fact I am still using that same flash and trigger system for my current Panasonic G80, just seems to work with any camera:
https://www.amazon.co.uk/Neewer-Cha...&qid=1550972848&sr=8-3&keywords=flash+trigger
I had 2x SB900 units [bought on here in fact] previous and sold them on a couple of Yongnuo units - for me, as a mostly manual flash user, there was no difference.
Other factors - I didn't miss the much larger Raw files, I never felt I was lacking in image quality [sure, smaller files, less crop-ability, but nothing major] - in fact I think the only thing I missed was the grip/ergonomics of the D800, it just took some getting used to and it didn't take very long. I did get a battery grip for the XT1, and I felt it benefited even though it was just the one extra battery for that model, just ergonomically I enjoyed using it more. I never ugraded to the XT-2 but often wished I had.
The 55-200 I can also vouch for, excellent lens for the money. It's sharp wide open at 200mm meaning you can rely on it in dimmer lighting, as it's got that nice 4.8 aperture and the OIS is excellent on it. I never shot planes with it but I cannot see that being any problem.