This is not an easy question to "answer"

The process of composing is a combination of theory and art. As you mentioned in the theory section, having a good knowledge of scales, counterpoint, cadences, harmony etc. is essential if you want to create a multidimensional work. But the real difficulty lies in the "art" of composing--putting everything together into something meaningful. I think becoming a well-rounded pianist helps the composition process as I think it's no coincidence that some of the greatest composers for piano and orchestral music were also fine pianists as well.
One trick to composing is to approach it like a poem. I think poetry is music's closest artistic "cousin" (hence why Chopin was/is called "the Poet of the Piano" and numerous "tone poems" can be find in the piano/ochestra repertoire). Before you compose, choose a form: sonata (good luck

), nocturne, ballade, barcarole, rondo, waltz, etc. Having an idea of the form will give you a framework. Next set the "tone" for the work: will it be tranquil, introspective, regal, eerie? Most works move through different often contrasting tones. This is a good technique to add depth to your work. Finally, write a melody and work off of that. Seems easy right?
One of the best things you can do is to analyze works by composers and see how they communicate their vision for the work. Take Chopin's Sonata #2 movement 1. It begins with a menacing intro and immediately enters into a dark, agitated section with a distressed, breathless quality. There's an urgency to get somewhere, and a galloping effect like a cavalry riding in a storm. How does Chopin achieve this?: minor key, a "short" time signature 2/4 meter, quick tempo, constantly moving left hand accompaniment, agitated melody, varied dynamics...
Hope this helped in some way.