It's hard to picture, the one issue you do allude to is the conduction
of the heat from the rest of the roof, into the area over the little
office. Does the attic in general have good airflow ?

In the summer you will also pick up heat from the garage, if it tends
to be hot.

If the room is really small like 6 x 6 it may just be easier to run
that A/C thing and be done with it.