What is better on the organ - playing the piece over and over without stopping or working in small fragments?
Every person is different, of course and for some simply just playing the composition many times means constant progress.
But for others such progress is often unpredictable because mistakes you make somewhere in the middle of the piece don't get corrected right away.
There is no immediate feedback loop: error-correction-error-correction.
Much more secure way is to work in small fragments. You repeat them many times in a slow tempo. When you make a mistake, the fragment is manageable enough for you to go back immediately and fix it.
Success comes in small fragments.