Hi ^^
The Discord server is Patreon exclusive yes, it's possible to discuss in there but its main purpose is for the monstergirl polls, which is why it's for patrons only!
Discord offers you various settings to differenciate between usergroups, for example you could have your patreons have a special role and a channel that is only accessible through that role.
I'm not sure how much work it'd be to verify patreons though, so I'd understand if you didn't want to go through the trouble