Just bring the storage space wall closer to the staircase entrance, to make the door & storage space higher, but also to allow more storage space. A lot of space in front of the storage on the picture have no practical use.
You also save some money on balustrades and side finishes of the staircase.
There is no reason you need to have architrave proud of the wall around the door. A good carpenter can build the frame and door flush with the walls. Paint everything wall colour, and just add some skirting to complete the look, and ugly little door is gone.
We have the same issue, with our understairs at the end of a corridor. I’m going to get the builder to make the door a hidden door (per arcadelt above), and wallpaper the door and the wall it’s in.
There is no reason you need to have architrave proud of the wall around the door. A good carpenter can build the frame and door flush with the walls. Paint everything wall colour, and just add some skirting to complete the look, and ugly little door is gone.