As you saw. the APR button will enable a glide path all the way to the runway. On a visual approach, it can be pressed as soon as you enter the DIRECT to the FINAL leg of the approach. Using it that way will replace the green VPTH in the PFD with a green GP right after pressing it.
If you ever activate an approach (cause a DIRECT TO the first waypoint in an approach sequence), APR can be pressed immediately afterward, and a white GP (pending glide path armed) will appear in the PFD and later at the appropriate location the NXI will automatically switch from green VPTH to green GP in the PFD without further interaction.
The rule is pressing APR will work only when in the approach sequence or on a DIRECT TO a waypoint in an approach sequence.
Note: I do see a magenta diamond when on the glide path, the same diamond I see earlier when green VPTH is active. VPTH is armed after reaching cruise altitude, ALT is set below the minimum altitude shown on the FINAL leg of the flight plan, and VNV is pressed.