There's this one person who claims to have found out that theses probiotics need to be amplified into a megadose, and subsequently they cured themselves from social anxiety. I'm not sure if there are other people who have replicated, but the initial report seems trustworthy. See https://pdfcoffee.com/experimental-treatment-for-social-phob...
That's very interesting. Making your own yogurt sounds like it could be easy to do and you could potentially just use OTC probiotic capsules or powder.
I posted some studies above about two probiotic strains that have specifically been studied for mood enhancement aka "psychobiotics": Lactobacillus helveticus Rosell-52 and Bifidobacterium longum Rosell-175.
I’ve seen people who seem to know what they’re talking about cast doubt that you could create your own probiotic yogurt in a typical kitchen since other strains are likely to dominate without a sterile environment and careful methods.
The strain you want has a huge headstart, and there shouldn't be much bacteria in boiled milk to begin with. Containers can be also sterilized with boiling water. Then keep it cool after it's done (14 hours). It doesn't seem likely that some other strain would dominate, but who knows.
Aside from the other red flags in that link, the author claims to be able to get rid of SAD symptoms in "about 45 minutes" which strongly suggests a placebo effect to me (or at least not an effect from changes in their microbiome).