In a bid to do anything but work on the dozens of projects I already have, I decided to make a userscript that allows searching pools by the tags on the posts in said pool
You can find the source for the script here, and you can click here to install the latest version
To use it simply install the userscript and go to https://e621.net/pools, you'll find a new "Tags" input, which also supports autocomplete. This search currently supports unlimited tags, and the or/not syntax (~/-) - it does not support metatags, and likely never will
Searches that use the tags field will not reload the page when submitted to avoid unnecessary requests to e621 (where we would just throw away the results), they will instead clear the results of the current page and replace them with the correct results
Any bugs/issues/suggestions can be said here, or in the github repo of the userscript, I fully expect this to break within a few days despite doing a generous amount of testing
API
Additionally, the api this uses is one of my own making, https://pools.e621.ws/json
It's updated via the db exports daily around 8:30AM UTC (e621's exports include everything between somewhere around 7-8AM UTC)
You can find the source for this api here
This api should accept any search parameter that https://e621.net/pools does, and they should work the same for the most part (timestamps might be a bit wonky)
Feel free to use it in external applications
For an idea of the structure of the response, you can view the types used in the userscript, PoolResponse is the root JSON