What's here is beautiful. Lovingly lush and serene. The rhythmic consistency is just the right length without feeling too sparse or overdone.
I guess what I mainly miss here is melodic motion, even though the harmonic motion is so great in this piece, just to match what's under. I do see what you were going for, I really do, what with the sweeping parallel chords, but it works as more of an opener or a bridge type situation, and doesn't give much for the audience to latch onto.
The other thing is that I personally (take that with a grain of salt) the Lydian mode works best when it intertwines with the Ionian mode giving the piece a sense of push and pull throughout. I had a lot of push in this piece with the raised fourth but never felt like I was on a roller coaster, more like those stereotypical swan love rides, if you know what I'm talking about.
Your harp writing looks generally good, save for a few sharp 4's here and there requiring a chromatics pedal, and the rest of the orchestration was generally good too. Use of oboes for the ostinato is a great color. Modulations were good, but I'd throw in some courtesy naturals/accidentals at the beginnings of some of them, especially 7th of B to tonic of Ab.
I know the feeling of being proud of a first part and not really knowing where to go from there, fearing a stagnation. Sometimes it's better to just go with it sometimes and see what happens. Worst that can happen is that you have a separate piece you can edit later.