The thing I used to use stats.wikimedia.org most often for is browser usage statistics.
However this with mixed feelings because I know it's user-agent parsing logic and how it's become dated.

I know we store the UA in Hadoop (right?)

Yes, and that's the source we'd use as we update wikistats reports.

and we preprocess it with the ua-parser library (which I trust). We just need a good dashboard to get continuous insight into this data. Is this data presented in one of the current analytics dashboards?

No, so far we've just run one-off jobs to get people answers to specific questions.

Right now I'm flying blind and this has stagnated many different projects and initiatives because I can't trust the data. Resulting in either questionable decisions, stalled issues, or using third-party data instead.

Thanks for pointing out that this is of high value. I have logged this here (the epic we'll use to keep track of new Wikistats work): https://phabricator.wikimedia.org/T107175