To the extent that you can do it without spamming, promoting communities themselves wouldn't be a bad idea, especially if you can offer a unique twist on an existing community or no analogue exists. You can reply to users who are looking for an off-reddit community. That way you attract genuine interest.

I think x-posting content is a good idea, but creating unique content, genuine content is better. Why would they come here if they can also see it on reddit? If there's content here that people are interested in that they can't find on reddit, they'll come here organically.