I don't freelance but have considered it in the past.
Why go to developer meetups? You will only meet other developers... go to marketing meetups for business people, young marketing executives etc.
Developers know other developers. But developers also know business people, startup aspirants, and other existing and potential customers. Developers can also have "overflow" where they have more work than they can handle and need to offload it. My own freelance career is supported by building direct relationships with clients and building relationships with other developers/designers/producers who I can work with on an ad-hoc basis.
In this particular case, OP's primary skill is "conversion optimization", which can complement another developer's skillset. That's a specialization that a developer can pass along to a client.
In this particular case, OP's primary skill is "conversion optimization", which can complement another developer's skillset. That's a specialization that a developer can pass along to a client.