So, you should take a look at probably the most famous orchestration of this piece by Caplet: https://www.youtube.com/watch?v=5DB8rBvyPoE
And I'm gonna suggest something really boring, but if you have an extra few hours, I'd go and like copy all of this—like, note for note. By hand, if you can, just to get it in your hands and brain what instruments go where and what they're best at doing (possibly even better if you try to transcribe it, then check your work). You've got some pretty good foundations for what makes an orchestral work tick, which is a great start (e.g., you mention adding more instruments to make the piece feel fuller), but you'll notice that there are so many times where what's written deviates from the piano score. Like at 3:05, when the main theme comes back in, Caplet adds these gorgeous violin solo notes to add chord pitches (that aren't even in the original!), a sustained note in a sing-song register, and serve as counterpoint. Also, going into 3/4 in half the orchestra at 1:00, while everyone else stays in 9/8? What a cool effect!
And the best part is, you'll get to see where you and he align on your choices! For example, at 3:08, you actually agreed that double flutes would work, but I believe you changed it to flute and clarinet in the final version of yours.
It's worth taking the opportunity here to also improve standard notation practices. Like in the first measure, in 9/8 you can't actually have any half notes because 9/8 is representing triplet compound meter, so three dotted half notes need to be visually represented. Little things like that go a long way.
You have a good budding ear for what makes orchestral works sound so pretty and grand, but you can take this opportunity to really reinforce that knowledge with score study and practice. The theory of it doesn't really matter (lessons on acoustics and stuff have always been super boring to me); what matters is knowing what things pair well and knowing when to use those tools. Best of luck and keep at it.