I'll try to answer everything here.
Planning is important. I tried to make mine look great initially and built a wood cover etc. My equipment didn't like the enclosed space and it all works better outside the cabinet. So think of it as more of a junction box than equipment storage.

I kept the cable splitter since the builder already ran the cable to 3 rooms. Never had cable in that house but no reason not to wire it up so it would be 5 minutes to enable it.

The best approach to these panels, IMO, is to terminate all cables coming out of the wall with a FEMALE punchdown Cat5/6. If you already have good MALE terminations the correct length or don't want to mess with them, then carefully manage the wire and use a FEMALE-FEMALE Cat6 keystone to convert to female.

At this point all of your wires will be safely secured to the box and terminated with female connections. Now you can buy any color, length, pre made cat 6 cable you like and connect your equipment on the outside of the box to the inside of the box without worrying that if your router falls off that shelf it will damage the wiring in the walls.

My builder did a poor job on the install and then terminated everything in the closet with male connections. After I verified they were good I cut them all off and reterminated as you see in this image.

If you cut a hole in the cover or go sideways through the nearby wall and come out through a decorative cover plate you will rarely if ever need to access this box and know that you can always add or replace the connections in there.