First 1000 users: daily manually done reddit posts. Very time-consuming and annoying, but it gets the job done. Just make sure the content drives users back to the site and is actually relevant, interesting, and valuable
Next 100K users: programmatic long-tail SEO. obviously this is unique to my own product, but I realized that people were organically already searching for the data contained within the maps I host. By focusing on organizing that data and making it understandable to Google, I started a traffic flywheel that's paid off massively.
I'm now exploring programmatic social media marketing as the next lever for the next 1M users as it directly drives even further benefits on the SEO side
One last thought - whatever growth channel you pick should really align with the product you are building. Some products are a great fit for SEO, others not. Some are awesome for Tiktok/Reels, others not. I don't think there's a one-size-fits-all solution.
Good luck!