My strategy for the past couple of years has been using RES [1] to browse /r/all, filter out everything I consider a waste of time (irrelevant, toxic, or just dumb) and subscribe to the few subreddits that caught my eye. This has had limited success - it's a lot of work, but it did surface some gems. I'm sorry to say it's literally like panning for gold.

* [1] https://redditenhancementsuite.com/