How do you get an open-source product noticed by developers?

In my experience, the developer tools that really catch on do so via word of mouth. For example, our whole team recently adopted https://sourcery.ai/ (not an ad) because one developer tried it and hyped it up to everyone else who also liked it. Developers are the best kind of customers if you have PMF - they evangelise and actively enjoy debating toolchains.

The hard part is usually building the actually valuable tool.

Hard Work… and more hard work.

Developers are mostly not looking for new products so it helps (a) to have something really remarkable, and (b) bang the drum louder and much more often than you think is necessary to draw attention to it.