1) Your VSTs are awesome
2) Your harp part does have some significant problems. Why are those arrows on the top of the barlines like that? That's not where it goes. They should be next to the notes. By default, the gliss goes up so you don't need to do that. It's already understood.
Additionally, you have some five note parts in the right-hand which is a major NO-NO!! The most notes the harp can play at one time is eight, not ten. Also, you don't want to stretch hands generally more than an octave. Tenths are a pain especially when you add other notes in it which don't allow the harpist to play chords easier.
3) Melody I see is NOT your problem. They are gorgeous and so catchy. That is a gift. Add some counterpoint to strengthen your melodies. Maybe a clarinet during that harp solo? Countermelodies (CM) are missing in this overture. You actually won't need to write something new because you have enough melodic material to borrow from.
Even in this theme, he develops density although there's not a strong CM in the first introduction of theme. but notice how he changes the color with the flute. The CM is faint but the movement in the strings and harp are clear. At 2:29, the horns have the CM. It's VERY present. Then at 3:02, it's VERY present as he builds density by adding the trombones. This is why it works. The themes are clearly developed. Even in an overture, the themes are developed; it's not a snapshot.