More AF points won't necessarily give better tracking, the only real advantage is IF your subject moves close to the edge of the frame. Smaller package with a prime? Yes, a little but it always surprises me how light the D750 and 50mm f1.8 combo is on the rare time I use it. Higher FPS can be an advantage for sure. I don't think you'd be wanting to use the silent shutter indoors tbh due to the banding in artificial light.
The question is, is the slightly smaller package, wider AF spread, and FPS worth the £2k+ it'd cost you to change bearing in mind your final image will be no better, and arguably worse if you don't fix the colours.