Looking for early testers, any ideas?

I've built an app that I use for my own freelance business (https://heyslate.com) and finally got it in a position that others can sign up to use. I'm trying to reach out to other freelancers (designer/developers) to test it out but I'm a little unsure how to approach them.

I've reached out to people I know but would ideally like to get some feedback from people who won't have a bias because they know me.

Has anyone got any ideas or experience in this phase of their startup that might be of some help regarding cold outreach?