Your submission proves that improvisation can be performed in any type of music! Very interesting. Your ear led you nicely through the harmony, which shows you are developing that connection between ear and flute. Your tone and intonation is beautiful. I am assuming that your actual improv starts around 57:53? Sounds like melody before that.

Let's consider your point about quarter and half notes... This is obviously a piece of music and an environment (mass) that doesn't call for burning fast runs or sophisticated harmonic lines. I think you are right to keep things more simple. Depending on how expressive you wish to be, there are a few things you can do. Notice how closely the rhythm of your lines follow the harpsichord (?) and the choir. You might try to separate yourself a bit by playing rhythms that compliment rather than follow the underlying music accompaniment. Play higher sustained notes in places. That also adds emotion. You have around 80 seconds, so think of that time as a musical arc - beginning, middle, and end. So build your solo by gradually adding these elements of complementary rhythms and range, and note length to build a complete musical statement. For example, when your improvisation starts, you could play a rhythm or a run that signals, "Here's something new and different."

The music over which you are playing is like accompanying a single vocalist in a band. When the choir enters, your solo can stand out more my playing something complementary to the rhythm of the choir. Their sustained notes are an opportunity for you to play something that contrasts rhythmically rather than mirroring their rhythms of quarter and half notes. Their sustained notes are windows for you to play something that stands out.

Last, think of your note choices. Much of your solo sticks to the basic chord tones. Now, this might be exactly what is called for. Again, your task is not to play harmonically complex lines. But, peppering in the ninth or seventh of the chords over the choir sustaining other chord tones can add interest and emotion. And despite all my usual advice about not running scales, this is the exception. A well-timed run up of scale tones (8th or 16th notes) to an interesting sustained non-triad note would also add interest and emotion. But, again, I want to tread lightly here because this is a sacred event, but I do think there is room for more rhythmic and harmonic interest you can create. I think that will answer you concern about playing primarily quarter and half notes.

See below where I created a play-along track to practice all this. Over this track, you get more adventurous!

Thank you for sending this. I would have not considered this type of music, but I think you did a very nice job. Now "season to taste!"