The comment-first strategy
The advice 'contribute before you promote' is correct and useless, because nobody tells you what contributing actually looks like day to day. This does.
Pick two communities. Not six. Two, where the people you want are actually present, and run this for fourteen days.
The routine
1Fifteen minutes, once a day
Set a timer. This is a maintenance task, not a project, and treating it like a project is how it gets abandoned by day four.
2Two comments minimum, on other people's posts
Answer a question, add a detail, disagree usefully. Never mention your product. Not once, not subtly.
3One of the two must be genuinely useful
Something only you know: a gotcha you hit, a number from your own work, a link to something that isn't yours. 'Great post' is not a contribution, and everyone can tell.
4Note the names that keep appearing
Every community has ten people who set its tone. After two weeks you'll know who they are, and whether they've replied to you is the actual measure of whether this is working.
Then, and only then
- Post in whatever the designated place is, if one exists.
- Open with the community, not the product: what you learned here that changed what you built.
- Ask for something specific and small. 'Does this solve the thing three of you complained about last week' gets answers. 'Feedback welcome' gets nothing.
Why this works when nothing else does
Communities are not distribution channels that happen to have rules — they're groups of people with a shared low opinion of being marketed at. Two weeks of comments doesn't trick anyone. It makes you a person who is there, which is the only category of poster that gets read.
The strongest predictor of whether a launch post gets engagement isn't the post. It's whether anyone in the thread recognises the username.