Short answer: in blinded, controlled studies, microdosing has not consistently outperformed placebo. Expectation and placebo effects likely explain a large share of reported benefits.
On mechanisms: tryptophan has limited brain uptake, while psilocin from psilocybin crosses the blood brain barrier more easily. Psilocin and serotonin share MAO-driven breakdown routes. There is no solid evidence that oxidized psilocin or its metabolites are converted back into serotonin precursors in the brain. I cannot verify claims that microdosing boosts serotonin by recycling psilocin.
Why do some people report lasting positives? Possibilities include placebo, set and setting, behavioral changes, and indirect mechanisms suggested by higher-dose research. Evidence specific to microdosing remains mixed and preliminary.
If you are comparing approaches, some argue that full-dose work shows clearer effects than microdosing, as noted in this overview. For now, treat microdosing claims with caution and rely on careful self-observation.
Hope this helps.