Matrix is IMHO a bad choice as it attracts the same demographic as Discord (glossy webclient) but is much more janky. Realistically speaking it is a poor Slack clone once you look beyond the technical aspect of federation which few people care deeply enough about to endure the buggy and half-broken user experience of Element.

I have had great success bridging to IRC (and XMPP). Yes, it will not fully replace Discord, but it allows a very dedicated group of people to participate in a community on their own terms and with great lightweight clients.

I agree though that any kind of bridge increases the risk of spam. But you should really try to get community members on board to deal with this kind of thing. Developing a software and running a community alone is not a good idea.