Ok. The D750 is kinda off the table. All of your lenses are really meant for DX bodies, except for the 50 1.8. So if you could live with just one lens, then a D750 is fine for what you shoot. While it's true that you can use a DX lens on a FX camera, you will essentially cripple the full frame body. What happens is the image you see in the viewfinder will go dark around the edges and the camera's resolution will be cut, since the image circle formed on the sensor is smaller by using the DX lens. Believe me, it's no fun seeing what you are "missing" in the viewfinder when you use a DX lens on a Full Frame body. It's one of those things...just because you can, doesn't mean that you should. That said, you could use Live View on the newer camera bodies, which does show you the image on the LCD, without the dark edges. There are pros and cons to using this method, as using Live View can drain your battery a little quicker and if you are really shooting fast moving objects, ideally...you want to be using the viewfinder for that sort of subject. But Live View is useful in a pinch and some even prefer it, especially when shooting video.
The D500 might be overkill. Honestly, you need better lenses than a new camera body. If you really want to improve your image quality, start with lenses. What don't you like about the 35 1.8? Is that the AF-S 35mm f/1.8G DX lens? If so, I love mine. What problems are you having with it?
Oh, one thought. Be careful with believing the marketing hype with the sample images. I have a friend who had a image used by Nikon for a commercial. It was to promote the then new D3300. Guess what her image was taken with? A Nikon D3s and 24-70. The ad actor said, "You could take this image with a Nikon camera..." So they weren't "lying," but not telling the whole story. Don't think for a minute that those D500 sample images were taken with a 18-55 kit lens.