Very interesting discussion! Here is another recent article that challenges the serotonin theory of depression. The authors of this meta analysis concluded, “there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity… Most studies found no evidence of reduced serotonin activity in people with depression compared to people without. High quality, well-powered genetic studies effectively exclude an association between genotypes related to the serotonin system and depression, including a proposed interaction with stress.”
https://www.nature.com/articles/s41380-022-01661-0#Sec13