I agree with you that downstream effects are immeasurable, and I wouldn't want to go down that particular rabbit-hole for every substance, drug, or supplement. However, the effect seems to have been profound in my case, and identical to the one I experienced from 5-HTP so in this particular case it's meaningful to me.
To extrapolate a little further, though, I actually wonder if what I was experiencing was not directly related to serotonin levels, but indirectly related to immune function. What I experienced could go along with serotonin syndrome, but also could have been a cytokine storm. So, not linked to increased serotonin per se, but linked to alterations in immune function.
There were honestly too many variables in my acute phase to be able to say. I did have Bell's-palsy-like symptoms following, with the right side of my face slumping for ages.
Hate even thinking about those times. My heart is with everyone who is still there.