Upgrade options: Fuji X-T4 vs Nikon Z6

I have a Fuji X-H1, and a nice set of Fuji lenses. I love the Fuji colors, and increasingly I am actually shooting for the JPEGs almost as much as the raws - great colors and much less work in post-processing.
So I am not in hurry to upgrade, but I am considering upgrade options.  I shoot a wide variety of photography.  I do some event shooting, I do a lot of bird and wildlife photography (I really love that), and some studio portraits — I am considering going semi-professional (doing some paid work).  But right now, I mostly shoot for pleasure.
My upgrade decision is driven by my bird photography in particular -  I would like better autofocus.  The X-H1 is good, I use the medium sized zone in the center of the sensor where the PDAF points are located, and the camera does alright as long as my subject is in the center of the frame. But I would like my next camera to have better autofocus, with the points across the whole frame.  The Fuji X-T4 sounds attractive in this regard, but so do some other cameras. 
My other wildlife photography  concern is relates to high ISO performance. I shoot at a high shutter speed to freeze action when I am shooting birds.  However, the results in high ISO pictures in most scenarios, unless it is a very bright scene (I usually set ISO to auto, capped at 6400). Would shooting with a full frame camera help me keep the ISO lower, and reduce the level of noise in my bird/wildlife photos?  Is improved ISO performance worth a potential sacrifice of those amazing Fuji jpegs?