The main point Ultima is making is that you don't need anything fancy to make good danmaku - in the Danmakufu community at least, we've come to the point where originality and creativity are key.
Yes, you *can* use extremely complicated math to develop danmaku, but any practical application of given math will require further manipulation of the given forms in order to account for originality, effect, user experience, aesthetics, etc. Personally, the more complicated the math, the less likely people will like it because the essence of many mathematical curves is not necessarily compatible with the overarching themes of danmaku.
From a research perspective, you obviously want to explore possibilities outside of basic trigonometry and standard acceleration, which, unfortunately, is the foundation of danmaku in Danmakufu. Your question states that you specifically want to present a theory in the scope of Unity, but that does not change the fact that your focus is simply spawning bullets in mathematical patterns. Spawning is limited. What transformations and the way you spawn the bullets is what really creates 'good' danmaku.
Of course, I'm just rambling from the perspective of someone who has seen so much ineffective danmaku that of course, I am speaking from an application standpoint rather than simple theory. But at least in Danmakufu, parametrics, polar equations, rotation matrices, and other trigonometry-based methods of spawning bullets are the most complicated mathematics-based patterns usually get.
Sorry I can't be of more help.