Some of the most devastating music i’ve heard still yet contain a mixture of the minor/major mode but commence and return to the minor. There are some outliers though in your spirit, such as Bach’s crucifixus (from mass in b minor). I believe that is entirely in the minor, but with an important caveat: The movement is approximately 3 minutes long, and is immediately followed by the jubilant ‘Et Resurrexit’. This is of course highly effective, and I can only imagine how the mood in a concert hall, reduced to the depths of despair at the depiction of Jesus dying on the cross, to then be elevated by the sudden and ecstatic news of the great revelation. The other example that comes to mind is Dido’s Lament by Purcell. Both share a the same structural basis: a repeated ground bass and as such the music remains within its harmonic frame. And both represent something both devastating and apparently defeating. So it’s not unprecedented in that measure, but what sets your work apart is in its length, and the indication that the other movements are also predominately in the minor. I would argue that the most effective narratives through music embody some form of cyclic structure that involves both the major and minor (not in equal measure but 70/30, depending on which mode the work commences) and if not within a single movement, then between movements. Bach’s crucifixus is an excellent example in this respect. That is my personal preference, of course, and wish not to detract from the technical and aesthetic feat you have achieved here. All the best Markus