Long day, finally back. The key was a combination of (a) identifying the right audience, (b) finding a distribution channel that could consistently reach them in large numbers, (c) creating a product ideal for that audience and distribution channel, and (d) parlaying that traffic into its own community. And not necessarily in that order.

The answer to those questions turned out to be (a) developers who want to self-fund profitable online businesses, (b) Hacker News, (c) interviews with successful developer founders willing to tell their story and share revenue numbers in the open, and (d) getting readers onto a mailing list, then sending them links to interesting forum threads every week for a year.

Lest I seem smarter than I am, this analysis is being done with the benefit of hindsight. I did not have 100% of this stuff planned from the beginning.

I put my whole history up here: https://www.indiehackers.com/product/indie-hackers. I've got to run, but hopefully will remember to come back and edit this comment to respond to some of your questions!