My advice is to never rely completely on muscle memory. Although muscle memory very palpable and direct to a pianist, one needs to reinforce it with other forms of memory: aural memory, visual memory, tactile memory, and cognitive memory. Think about this statement: you cannot memorize something you do not know. To elaborate, in order to access these types of memory you need to make a conscious effort to observe and collect information relative to that form of memory. If you haven't done that data collection in the first place, there is nothing to store in memory. For harmonically complex slow music, observe the shapes of the chords on the keyboard. Do this with the score in front of you. Look down at the keys every so often and observe what is happening. You need to make these observations in order to store data on visual details. Run through the piece numerous times with the goal that with every repetition you will direct your visual attention more to the keys than to the score. Eventually you should be able to get through 100% of the piece only looking at your hands. For tactile memory, make note of how a chord feels in the hand, especially complex chord structures. Try to replicate chords in other octaves of the keyboard. For harmonically rich music, it is also helpful to somewhat understand what the harmony is doing. Think of the basic V - I cadences and sequences you learned in harmony. These things still exist in more complex tonal structures, but think more in terms of relatively dissonant chord resolves to relatively consonant chord. Also observe instances of counterpoint and thematic development.