But yeah but if the setting is right what the hit rate
Andy I think you might be over thinking it for a change
Looking at your photostream you're not taking shots that are demanding on AF systems therefore you should be fine whether you stick with the D750, get another A7, another Fuji, or decide to try something completely different and get a Leica
If you were heading down to Donington every week, or going to events like this Wednesday's Sheffield Grand Prix, or wanting to take pics of erratic birds in flight then I'd be telling you to stick with the Nikon.
As it is with the shots you've taken to date any camera will be fine. Instead of worrying about absolute critical sharpness focus (excuse the pun) on the more important things like what you're capturing, getting the 'moment', composition etc. We all want our images to be tack sharp every time but in reality it's not the be all and end all, some of the best shots I've seen have been quite soft, but it's not mattered as the images were powerful and told a story.
Also, when looking at cameras focus on ergonomics, button placement, lens selection. Cameras are about having fun, stressing over critical sharpness should not take over that enjoyment. That's not to say it should be neglected, just don't let it over shadow everything else.
You've gone from a 5D3 to D750 to Sony A7 (Can't remember which model), back to a D750 (with possibly a Fuji in between) and they can't all have had focus issues
Do you think maybe you should look at what you might be doing wrong rather than looking at changing cameras every time you get some shots you're not happy with