This looks pretty good! I mostly have some playability-related comments on a few specific parts:
-m28: It feels kinda weird to me to put the D# in m26 in parentheses for the tenth interval in the R.H., but not the C here for the L.H. though in this case, you do lose a bit of harmonic information if the C is optional. After some thinking, you could add a C in beat 1 and a D in beat 2 in the R.H., so a bit more notes of the L.H. are doubled, which makes the C in the L.H. optional:

I can hear the C and D I added in the R.H. in the original too btw
-m35: These E's in the L.H. are a bit stretchy too to not be notified as optional, but I think it might be better to move them down an octave, so they're more in the same area as the rest of the L.H. in this section, and follow the same voice leading as in m43.
-m38: It might be a bit better to start the 8va at the two 16ths in m37, so there isn't a sudden octave jump in the middle of a phrase. It makes the phrase easier to read. Make sure the 8 of the 8va starts a bit before the first note too btw, it's currently a bit too much to the right.
-m39 and 40: The R.H. half note in m39 and the L.H. half note in m40 aren't possible to hold while playing the other voices without using a pedal. Using a pedal, however, takes away the staccatoness of the flute and violin notes. There's a few things you can do:
-Still use pedal and use the staccatos in the R.H. more as a sound thing (mostly noticeable on acoustic or grand pianos). In that case it's best to add a pedal marking.
-Shorten the half notes to 8ths, which will cause nearly all notes in these two bars to be short (my least preferred option)
-Take some arrangers freedom to imitate the general contour of this part, while keeping the long and short notes (mostly) intact, for example something like this:

I edited the second 8th in both bars to a single note which is still reachable by both hands, and shortened the R.H. and L.H. halves to quarter notes, so they're shorter but not too much shorter.
I actually think the first option might be my preferred option haha. In all options, the L.H. D in m40 can be moved down an octave either way, as there won't be any stretching issues with the other voice.
-m53: The D# and E in the R.H. should be 16ths on beat 2.5, not triplet 8ths.
-m57: This R.H. 16th run is pretty hard to play, and doesn't really convey the flute melody that well, since it's mostly buried underneath the string line. Here's an alternative idea, moving the flute up an octave, and adding a bit of the string line in the L.H.

You could add the A in the R.H. on beat 1.75, but I felt it might be a bit hard to play with everything else going on while not adding that much effect.
-m60: You can add the harmony flute G and F# on beat 2.5 in the R.H. here too, like in the pickup measure.