Have a look at https://lemmy.world/c/newcommunities . I just know it exists and have got looking at it's rules and checking it out more on my to-do list.
you can promote your community by responding to relevant posts or comments.
say you create a community for ducks and mallards, then you can go to the birding or wildlife communities and tell people posting there about your awesome new place
You can always crosspost posts from your new comm to other comms as long as they're relevant to the comm you crosspost to. This is essentially advertising for your comm without making "pls post in my comm" posts