What is the best marketing approach I should use for my tool? (besides product hunt)

Quora could be useful. It is commonly used by freelancers. Otherwise, as Maxwell mentioned, try to look for relevant communities about freelancing and engage there. Running some outreach toward influencers could also be nice. You can try LinkedIn for it. Comment and contact people active on the freelancer hashtag: https://www.linkedin.com/feed/ha...

If this is competing with existing platforms like Fiverr, Upwork, Freelancer then personally I'd focus on building brand awareness but engaging in conversations in and around the freelancer space. That for me would be primarily Twitter and LinkedIn (FB may have something too just not that aware of it). I'd also think about developing some lead magnets: Top 10 tools for freelancers... How to start being a Freelancer Top ten mistakes a freelancer makes.. Something like that as a download that can capture an email address and then utilise an email autoresponder to convert to a customer. Maybe an opportunity here if the API is available would be to bring in data from the other platforms so the freelancer has a total view as I know that a lot of freelancers that use those platforms are on multiple ones.