Hi and welcome to the forums!
That's a decent chunk of material. Before diving in it, let me ask:
Why 17?
And now, to the score:
• Not a harpist, but this seems pretty uncomfortable to read (or at least one could say it has room for improvement in the engraving field) and the last bar has redundant silences. Apart from that, see how the arpeggio indication overlaps the silence in the first bar of the image.
Actually, the same goes for the piano staff. In the very first bars my guess is that you wanted to make the bass continue thorough the first four bars. You might already know this but in case you did not, you can engrave this by using the Sost. Pedal which does exactly what you want to (maybe not in playback depending on the software you're using). I would also recommend the use of 8va. bassa in certain parts of the score such as the ones just below the harp part I took a screenshot of.
• I suppose the first theme ends at M66; It didn't really convince me because:
The question-answer structure you try to make at the beginning sounds too disunited to me. There's no question —to my ears— neither answer, and specially the end of each answer (the mordent) is very unsatisfying.
There's barely intervention from other instruments.
The register in both the harp and the piano is kept low in the whole passage, but this might not be necessarily bad if the phrases were more convincing. Not the case in my opinion.
• The sound in the playback doesn't help the second theme AT ALL, but that's independent of the score. For what I'm listening to up to now, this seems more like a piano concerto than a symphony.
• There's much chaos, lots of voices, each one wants more prominence than the others thorough this second (or already the third) theme. That often goes against the piece and not in its benefit.
• I believe the material you bring near minute 6 is really good. It needs polishing though, as well as the whole score actually.
• Again, not an harpist, but this is wrong and absolutely not idiomatic to my knowledge:
C clef? Why? You put it twice in the staff without clef changes in between, too.
For how the harp is built (how pedals work), writing D and D# in the same chord is just not the way to do so. My recommendation: as I assume this is a work you would want to be as better engraved as possible, change those D# for Eb (the same for every other conflicting note); consider that the harpist needs to switch pedal positions for doing anything outside the natural alterations of the tonality, which is not really an issue unless you want them to do so during the execution of a single chord. For more info, I would recommend you to read Samuel Adler's "The study of orchestration", specifically the harp part of course.
• Sorry if I'm mistaken, but I am past the tenth page and I have not seen a single dynamic marking, which is almost compulsory in works of this density and length unless you intentionally want everything to sound with the very same volume. Summing that with the fact there are often lots of voices trying to get more prominence than other, this ends up as —in all honesty— a mess.
• I must insist: consider that we are talking about quite a big work, more than 20 minutes only the first movement and there's not variations in the volume of each voice overall. I haven't reached the tenth minute and I'm getting overwhelmed, exhausted and even a bit stressed already. I see neither a clear direction, nor significant chunks of good material as whenever they appear they rapidly get burdened by tons of notes of different voices that erase their momentum one another. Perhaps the only moments of contrast are the parts when there's barely any other instrument than the piano or the harp.
• The comeback to the first theme is a good choice in my opinion, regardless of the problems that lie into it. I would have ended the piece in M609, but instead you chose to continue and add another theme with poly-rhythms. All in all, the very, very last bars sounded interesting though replacing the left hand on the piano for a drum would have been a wiser choice.
In summary:
Very chaotic and unreasonably dense movement, with lots of voices trying to get prominence at the same time and no dynamics from what I've read.
Very questionable scoring: strange clef choices; non-idiomatic engraving for the harp; very messy piano engraving with —likely— redundant notes that get its voice dirtier and —also likely— nearly impossible passages; questionable scoring of the horns too, as in my opinion, more than one staff (just like you did with violins) would improve readability for the performers; notes, silences, clefs, etc. overlapping one another; badly split voices; too much unjustified "out of the staff" notes that may make its study more difficult (extended use of 8va. bassa and alta would be encouraged).
Despite its comeback to the very first theme nearly at the end of the piece, there's no recognizable structure other than one theme after another (I think, as some parts get too messy for me to know on a first and/or second listen) and linked together with more or less fortune. Probably, having a clear structure is not compulsory in a symphony nor in many other genres, much less abiding to a strict set of rules, but most of them have at least significant traces of one, and this is for a solid reason.
Perhaps you do not agree with me in anything I have commented and criticized here (though, there are some points that are not subject to opinions such as the harp engraving), and I totally respect it, but this is my honest opinion and I think this is what usually people desires to get.
Before proceeding any further with your piece, please consider revisiting this movement extensively.
Kind regards,
Daniel–Ømicrón.