I will also note that there has been some work done on the incorporation of Clostridium sporogenes into probiotics, but it doesn't come from kefir or miso, and... well... just read it! [3]
1: https://en.wikipedia.org/wiki/Tryptophan#Dietary_sources
2: https://jasbsci.biomedcentral.com/articles/10.1186/s40104-01...
https://en.wikipedia.org/wiki/Tempeh
I've never had Tempeh, but I love tofu, edamame, natto and other soy products. This is definitely on my list to try and explore the many varieties.
But Tempeh is a pretty common one for vegans. I've used it in curries and stir fries.
This leads me to believe that trying to increase indole-3-proprionate specifically may be a fruitless journey (pun not intended), and one would be better off by optimizing for a hospitable environment for beneficial microbes in general, for example by cutting out carbs but increasing intake of flavonoids.
I believe traditional salami also has a little propionate
This indole-3 link could be the reason why omega-6 oils are so bad.
TL;DR: Don't eat processed vegetable oils (other than coconut and olive oil).
The evidence against seed oils / processed vegetable oils is not very compelling yet (here's a pretty balanced summary: https://www.healthline.com/nutrition/are-vegetable-and-seed-... )
https://www.cancerschmancer.org/articles/healthiest-cooking-...
Annoyingly, it doesn't really pin down what exactly it characterizes as "the western diet". It does include extracted oils and refined simple sugars (glucose and friends). But where exactly stuff like pasta/rice/flour sit is less clear in their description.
However, alcohol is quite an irritant and (along with caffeine) is the first thing a doc will tell you to cut out if you have intestinal problems
Speaking (only) from personal experience, it can be such an irritant that it more than outweigs the benefits of other probiotics and "good foods" -- even in small amounts (e.g. 1 drink)