Although marked Presto, if one rushes due to the apparent ease of the beginning, the tempo may become unstable later on. In Variation 4, 16th notes appear in the left hand, suggesting that this might be the section where the tempo is most prone to decelerate. If this is the case, consider establishing the tempo of this section as the fundamental tempo and aligning the other sections with it. In other words, it is advisable not to rush from the outset. Maintain a consistent tempo throughout. Regarding dynamics, as the number of notes increases towards the latter half, the volume naturally tends to augment. Anticipating this, for the dynamics in the first half (up to the theme and approximately Variation II), even when 'forte' is indicated, it might be beneficial to control the volume rather than executing a full forte. Subsequently, from around Variation IV, gradually increasing the volume towards the conclusion can lead to a brilliant finish. For your reference.