Is OpenAPI 3 a requirement, or are alternatives an option for you? I worked with RAML 1.0 for a while, and it was much more expressive than OpenAPI 3.
That said, the tooling available for RAML 1.0 wasn’t that great, and I ended up switching to OpenAPI 3 for better documentation generation. (I’m not currently using any tooling for code or test generation, or other runtime support; the specifications are just documentation at this point.)
All of that was before I started looking at JSON:API, though.