Aa another Fuji v Nikon comparison, a few weeks ago I had both XPro2 and D750 and was surprised how much the AF-C on the Fuji lagged behind that on the the D750.
For me, the Fuji system is fantastic for many genres of photography but action / sports is, in my experience, not one.
I would also think twice about using a Fuji for weddings for that very reason; I would be worried about missing AF on crucial shots.
Those that do use Fuji here for weddings clearly have better photographic techniques than me - which is not difficult !
I must add that XT2 might be a tad better in this respect, I have no experience of that body.
Back to the OP, the D500 would be perfect for weddings, again, IMVHO. That said, FF would be best for the dreamy narrow DoF shots much liked by brides and their mothers.