THe A6400 should be a good choice. I seriously considered renting an A6600 plus Sony 200-600 last year for testing against the Fuji, but once I started playing with the Fuji 2X extender coupled with the Canon 100-400 Mk2 I decided it wasn't worth it as any gains would most likely be marginal. The sensors are basically the same so it would come down to Sony's AF vs Fuji's AF and Sony's ergonomics vs Fuji's. I have no doubt that Sony's AF is better, but Fuji's AF was/is getting the job done and I have gotten used to the dedicated parameter setting dials, most of the time.
I looked at the A6600 instead of the A6400 for the larger grip and bigger battery. I had experienced a Sony A7R2 a few years ago and hated its grip and the tiny battery that was good for less than 100 shots, typically. Battery life is a general problem for the MILCs, but I can usually get 150 to 200 on the X-H1 using the long lenses, and more than 300 if you are using smaller lenses and less chimping.
FWIW I picked up the X-H1 when Fuji was having their fire sale, $1000 for that camera plus vertical grip and three batteries was a steal.
