In my experience, your expectations shouldn't be as optimistic as success stories like that of Slack, which had 8000 companies signed up before they launched their product. It's wise to assume that your customer acquisition would be wearyingly slower. You don't have to start with 20-30 customers but just 1-5, who get the value out of your product.

For the last product, which I shut down due to lack of traction, the most valuable leads I received was when my product got featured in a newsletter without being asked. I had posted a comment on a Blog post which described a complicated GA setup to achieve something that my product could do without effort and the author forwarded it to his subscribers.

I think that sums it up quite well: look for people who have the same problem as you're solving and pitch them your product. Any other marketing effort: paid ads, blogging, events require too much investment and I don't think they should be recommended if you're bootstrapping a small product.