As a test I googled Steven universe mastodon and got this link: https://mastodon.social/tags/stevenuniverse where you can browse stuff people posted about it on that one server. You can then look around a bit following links with a web browser, without needing to sign up for an account.

I imagine Hubzilla is also public and searchable in theory, but there are fewer users. I've sometimes found links to peoples posts on Hubzilla when I've been looking into Hubzilla itself, and been able to look around without signing up.

Correction: I had a look at the Hubzilla page https://start.hubzilla.org/ and it doesn't look as easy to just web browse around the social network part of an instance as it is on mastodon. But it should still be much easier to find public discussions than on discord once there are some links to it. I don't think it would stay as obscure as semi-private discord servers.