1. How do you successfully build a community around a blockchain project without revealing too much prematurely before building a strong enough community?
You can't do that. A community is a group that believes in a project and trusts what it can become giving enough time and support, if they don't really know what the project is about, they cannot trust and build it.
What you can do is build anticipation for your project.
2. What steps have worked for you in fostering engagement and attracting early supporters?
Be creative.
There are thousands of similar projects looking to build a community, you need to make yours stand out. Identify where your target audience are, catch their atention and find a way to keep it.
3. When is the right time to start engaging with the community? Should it be during development or closer to launch?
During development ideally.
4. How can I establish trust within the community while maintaining security?
Both are not mutually exclusive. You maintain trust by being transparent with your community members and having an actual product which offers value.
5. What communication channels (besides Bitcointalk) have you found effective for community engagement?
Reddit, telegram, twitter.