Finally gave up googling and decided to ask people with real experience.
The free tiers on most of what I've tried are basically useless — you get just enough to see real people are there, then hit a wall the moment you try to message anyone.
I've noticed a big difference between platforms that moderate actively and those that don't. The ones that skip it become unusable within a few months.
If you've got experience with any platforms or approaches that work, drop them below.