Let's start with understanding the level of difficulty of an organ piece. You see, even though you like certain pieces by classical organ composers, the difficulty level of these compositions might be very different. So if you practice several compositions which are different in technical demands, then your organ playing skills will develop much slower.
Instead, you should find the system of using organ compositions in increasing level of difficulty. This will help you progress in organ playing in the step by step manner. What I recommend you would do is to find organ pieces which are easy in technical demands. For example, the keys should be simple enough, the rhythms should be no more than eighth notes, and the texture should have no more than three voices.
If you find that kind of composition, then you should master it and perfect it so that you can be ready for public performance with this piece. But you should not stop here.
Find similar compositions and learn them as well. When you feel that learning these pieces are getting easier and easier by the day, you can start looking for compositions which are more advanced. For example, they could have four voices, smaller note values like triplets or sixteenths and so on.
Perhaps the keys used in these pieces might have larger number of accidentals. Remember it is best to do it in a step by step manner. Increase technical demands of your piece just a little which means the requirements should not become more difficult in all these areas at once but only change one technical limitation at a time.
For example, choose a piece which has the same number of keys but smaller note values. Or it could be the piece with the same note values but with a different number of voices and so on.
Apply my tips in your practice and try to find these pieces. Mastering them will help you incrementally increase your organ playing skills.