Mastodon and Pleroma are Twitter alternatives, not Discord alternatives. The chat functionality for them is awfully implemented last time I checked. But then again it's a different target audience.
You are right. XMPP and IRC then.
Mastodon can de-platform you just as easily as Discord if they don't like what you say.
>Mastodon is run by "them"
That's not how federation works.
No more so than any random website. Mastodon is a federated platform, so find an instance that allows you to say what you want to.