First 100 users, meet them one by one wherever you can (forums, friends, ex-coworkers) and call them to talk to them and help them out with onboarding if needed. Straight up cold outreach and warm intros. Next 500 will most likely come from referrals if these first 100 users are happy.
Apply this logic to the jump from 20 to 100 if it makes the task less daunting for you.
Your current approach has specific techniques I would remove altogether to focus on the core interaction between you and the user: talk to so many of them so often that you truly understand their problem and how they shop for a solution.
Once you understand that and make them happy, you’ll come up with ideas to experiment how to get more users.
So, I am one of the users myself. I built it because I needed it. I ran it by few users in my network. We are constantly getting feedback from our early beta users, which are around 200. Not all of them are active.
My main problem is TOFU, which I need to solve for.
Apply this logic to the jump from 20 to 100 if it makes the task less daunting for you.