I need to reverse the rotation of the universal motor in question so I swapped the wires leading to the brushes and that successfully reversed the rotation, but now the commutator is arcing and is producing a significant amount of sparks. It's worth noting that the arcing completes half a circle almost touching both brushes simultaneously.
I tried rotating the brushes 180° because I suspected they were worn out "asymmetrically" despite looking straight (still concave but not unevenly worn) and without a slope but the sparking continued.
Then I sanded carefully the brushes so that they were squared again to sort of resetting them to their factory position so to speak but the arcing still persisted.
The only variable left is that the commutator is not perfectly worn and is making an undesired contact with the brushes.
I want to make clear that the housing of the brushes are not angled in any way to favour any direction of spin and to my understanding, it should be very feasible to reverse the direction.
I also tried sanding the commutator to clean it with fine-grit sandpaper but didn't want to go any further without consulting.
This is the stock configuration:
Stock configuration with commutator more visible:
"Custom" cables configuration: