Comment by reasonattlm

The 16S rRNA sequencing you are referring to is now quite cheap and efficient. [1]

Viome is one of many companies offering this as a service. Viome isn't so good at giving you the underlying data, some of the others may be better; look at their example reports or ask their support. Ombre, BIOHM, Tiny Health, Floré, etc.

In terms of whether the add-on services they offer are valuable (tailored prebiotics / probiotics / diet advice based on their reading of the literature) I think only time and randomized controlled trials will tell. Not that any of these services have any incentive to run trials! But it may be a useful time-saver for a self-experimenter who would have been digging through the literature to try to come to the same conclusions on things to test.

[1]: https://doi.org/10.1038/s41467-019-13036-1