If I am honest, I really don't think Nikon thought a great deal about "Flexible program" on the top end cameras. I think they expected the cameras to be used in Aperture priority or Shutter priority, Program was included because the chip set could do it rather than as a fully valid operating mode. I have to confess that I have never used Program on any of my cameras that have/had it, I see it as a rather unnecessary addition but if you want to use it, why not. I would guess that the D5 and D6 do these things in the same way as the D4, which is pretty much how you describe for the D800. I haven't so much as handled a Z series camera so I have no idea how they handle it.