I believe I already looked thoroughly, but just in case...have you found a way to make the D750 behave more like the Df with regards to having AF-ON on the rear of the camera but also keep the ability to use the shutter button for AF as well? From what I can tell, if you assign the AE-L/AF-L button as AF-ON, the shutter button no longer works for AF. It only works for metering/exposure and shutter release. I want the AE-L/AF-L button to be AF-ON just like the dedicated AF-ON button on my Df, but also keep AF on the shutter button too (Df will focus with either one, but obviously the shutter button only works for metering/exposure lock and shutter release if you're already pressing the rear AF-ON button). I think it's not possible to do this on the D750, but I thought I'd check in case somehow I'm not seeing the right setting.
The reason why I want it this way is because I normally use the shutter button for everything, but occasionally I want to focus and recompose and that's when I'd want to use the rear AF-ON button (so the exposure is not locked when I'm moved away from the normal framing for a moment to get focus). For the time being, I sort of do the reverse process since I can't make the rear button work the way I want. For now, I have the AE-L/AF-L button set to AE-L only and if I need to focus and recompose, I'll actually lock the exposure with the framing I want first, then move the camera way to focus where I want using the half press on the shutter button, then recompose (while still holding the rear button and now also holding the shutter button half way) and take my shot. That works ok, but it's not ideal.