How effective would it be to have all the doors on the upper floor fitted with transom windows? Does this really increase the amount of light, or is it more decorative?
My parents have it like that. Corridors without windows do actually gain light this way, although I found frosted glass doors to be more effective. BUT - if daylight enters the corridor through the transom windows during the day, at night light also spills from the corridor into the rooms. At the time, I found it extremely annoying that whenever someone was in the corridor, it got bright in my room.
Yes, I have also thought about that, but dismissed it for the same reason...
An alternative: Continue the "staircase recess" up to the exterior wall and install a window there. In my mind, however, that looks quite strange and the light would have a very long way to go...
It depends on the use of the rooms. The small room upstairs is not intended as a children's room, but as an office, right? Then make the doors of the bathroom and the office frosted glass doors (there are also modern designs). There are windows directly opposite the doors, so light should come through.