I'm fairly certain that the last 5 minutes of baking would not have opened the texture of the interior.
Pâte à Choux is supposed to be very soft inside. It has ribbons of eggy material running through it. That is why cream puffs are usually scooped out before filling.
The material is very tender so, with eclairs, injecting the filling pushes it out of the way and the shell is firm enough to hold in the pressure.
When the dough is baking the steam inflates the abundant egg proteins in big pockets. This happens in the first part of baking and the rest of the time is setting the proteins and drying the exterior. If your interior truly has as tight a crumb as bread, this would have happened at the beginning of baking and the 5 minutes at the end would not affect it.
Pâte à Choux is very easy to make. I wonder about your process.
Recipes always consist of:
- Combine and scald, or boil, milk, butter, sugar, and salt,
- Add flour all at once and stir until it forms a cohesive ball that pulls away
from the sides of the bowl,
- beat in the eggs one at a time.
I would make sure that your milk mixture was properly scalded and that your flour was mixed in well enough before adding the eggs. The flour should have gluten activated and its starches gelatinized before you add the eggs.
If you haven't been already, you might try using bread flour and see if the problem is lessened.