From what I understand deliberate practice would generally favor the small number of hard problems, especially for building overall mathematical competence/​your ability to tackle hard problems. However, doing the easy problems in a challenging way, like trying to do them as fast as possible while still maintaining a high standard of accuracy, would also lead to improvement, particularly for you ability to do that specific type of problem quickly and accurately.
From what I understand deliberate practice would generally favor the small number of hard problems, especially for building overall mathematical competence/​your ability to tackle hard problems. However, doing the easy problems in a challenging way, like trying to do them as fast as possible while still maintaining a high standard of accuracy, would also lead to improvement, particularly for you ability to do that specific type of problem quickly and accurately.