Hi I'm one of the founders at Permify. Thanks for bringing up the comparison. I’ve actually put together some documentation[0] that outlines the differences between various Zanzibar implementations, including Keto, OpenFGA, SpiceDB, and Topaz. Planning to add Warrant and others soon as well. Hope that helps.
[0] https://permify.notion.site/Differentiation-Between-Zanzibar...