That should only be an issue for media though shouldn't it, since all plain text can be set up to be e2e.
It does not matter whether you encrypt illegal content, things do not magically turn legal once they are encrypted.
Plaintext cannot be set up to be E2EE, that is a contradiction in itself. Rooms can be either unencrypted or encrypted. If they are, messages are encrypted.
I hear that hosting a synapse server is a pain in the ass, but I haven't gotten around to it yet.
Indeed, it is. That's why I have replaced mine with Tuwunel, much less effort and way less resource-intensive.