Early users rarely come from ads. They come from threads where someone just wrote "is there a tool that does X?". Here is a routine that takes 20 minutes a day.
Not keywords: sentences a buyer types. "alternative to Notion for…", "how do you handle invoices as a freelancer", "looking for a simple tool to…".
Use Reddit search sorted by new and Hacker News Algolia search by date. Threads older than 48 hours are mostly dead; the asker has already chosen.
Give a real answer in 3-5 lines. Mention your product only if it truly fits, and say you are the maker.
Track thread, reply, outcome. After two weeks, keep the phrases that brought replies and drop the rest.
Doing this by hand every day is the hard part. ThreadRadar runs the search daily and drafts the reply for you to post from your own account.