Gut microbiome work is an attractive topic and something that gains much publicity. I am personally hesitant to use the word "undeniably" due to my physician-scientist nature. As you discussed, there are promising studies suggesting an association both in rodent models and human models (https://bmcpsychiatry.biomedcentral.com/articles/10.1186/s12888-020-02654-5). This review paper I am citing critically appraised studies investigating fecal transplant with respect to psychiatric conditions. I think it is unlikely that fecal transplants will become mainstream for treating psychiatric conditions given that there are already many therapies that work (which insurance would be more likely to cover). Studies like these are still valuable because the identification of additional targets for therapy or potential nutritional changes could potentially cause a reasonable effect. I get the sense research is still in the exploratory phase vs robustly testing phase (the systemic review I read had the air about it suggesting it relied a bit more than desired on retrospective cohort studies, which can be a limiting source of information). These coming years will likely involve well-designed studies on this question.