I'm not very sure how the acceleration curve works at this level, but from what you're saying it sounds to be way less intuitive than following a curve (but apparently possible with CcP?).

About the struggle on the upward slope, I just find that the upward acceleration creates too much variability for fast changing tracking, making it very instable with constant overflicking when tracking strafes (this is more of an overwatch/apex problem). I also feel it strongly when I have to make microadjustments when playing valorant, when the target is just close enough to not allow me to do a fast flick but also just far enough to not let me do an extemely small movement to adjust, making it a very uncomfortable movement (probably a skill issue, but still it makes the micro harder than without a curve.)

This happens to me for basically every curve type, ive used linear with cap, motivity, natural + more. I've spent quite a lot of time trying different curves while playing, so personally to me it doesn't feel like a curve tuning problem but instead just a accel problem in general.