You can also check out specialized bots directly - top.gg has a dedicated section for game and riddle bots, you filter by category and check out the reviews. Stuff like Trivia Bot or more niche variations do the job pretty well, but honestly the real hack is to join a server with a good moderator and pitch your own riddles. Well-run communities always need fresh content, so that's the best way to find good stuff - you create quality, you attract people who want it.
What really works too is private channels or invite-only servers. A lot of enthusiasts create their own riddles instead of recycling generic ones, and the vibe is way better when it's restricted. You just need one person who knows someone. Like search Reddit first (r/riddles, r/puzzles) - you'll see who's organizing events or live Discord challenges. Those people usually have quality servers where you can apply.