Phew, that was really nice. Beautiful! I really like listening to improvisational pieces because they push my own improvisation in new and interesting ways.
It's funny because just yesterday or the day before I was wondering about the iii chord and I was thinking how I didn't use it all too much, so I was trying to figure out how it should work in a song. I thought you used it very well.
Thanks for the thoughtful comments! I used to always wonder about the iii chord as well, but there are a couple of progressions it works really nice in. EX:
I iii IV I
I iii vi IV
One of my favorite things to do is replace the iii with a dominant seven starting on the same root. For a somewhat bluesy progression:
CMaj E7 FMaj F#Dim CMaj/G G7 CMaj
or
CMaj E7 Amin FMaj
Another good way to use it is as a passing chord. For example:
CMaj Emin/B Amin
Hope some of that is useful to you!