I think you should get more sincerety exp for the harder difficulities. Because, getting only 5 for all the levels is kinda asking you to only grind the normals levels, as the harder levels take more ink for the same amount of sincerety. And this can be counter productive as you may need to grind the harder levels for certain materials.
So I suggest:
- Story and Normal Mode - 5 Sin Exp
- Hard Mode - 10 Sin Exp
- Very Hard Mode - 15 Sin Exp
To me this way it will feel more rewarding doing the harder levels