I have started a community and used discord. I had someone else set up the discord channel at the time but with this article I could have easily done it myself (it was nice to have help at the time though). I am more in charge of the promotion and you have got it right, I write posts and advertise the server and community. I also read the categorises relevant to my community and invite people when I comment on their post.
Rules must be obeyed! We are just starting to have to reprimand members for breaking the rules. It is a gentle approach at the moment but I can see we will have to get harsh as the community gets bigger. I am seeing members 'regulating' on our behalf now which is useful.
When they start telling others the rules it means that they value membership and that they clearly understand the rules, congrats!