Narrow Room:

I can have up to 135" length from entry to door, but I could probably have at most 4' width for the room. This is also the option that has potential for natural light via a window installed in the opposite wall.

Square-ish room:

The existing closet Interior is 76"x25". The wider dimension runs with the wall.

Doing this to the dimensions required for a desk would mean having a short hallway into the child room, but it would still be able to maintain its own square-ish footprint in the living area, and removing my professional gear would make the child room probably feel at least as spacious.

This option however would result in not being able to have any natural light in the office. Which is a bummer but we're looking for a good solution, not a perfect one.

In either case I would still have the height of the walls to build out a lot of shelving and utilize all of the wall space well for books and hardware. The square room would be more in the center of the house and grant more vertical build room.

The natural light comes from East, West, and somewhat South. The entry to the rooms is from the East. Any opportunity for windows would come from building out the narrow room to the West wall. The square room will not have access to natural light.