There is math stack exchange, and there is also mathoverflow, which is for more advanced math questions. And although it is not on the stack exchange family of websites, there is also a physicsoverflow website. Is there an analogous website for research-level philosophy questions, a "philosophy overflow", although it may not have that exact name. I apologize if this question is inappropriate for this forum. Perhaps it can be migrated to an appropriate stack exchange if it is.
No, there's just one site, like there is for most academic fields. We get lots of more introductory questions here, but also lots of more advanced questions. For sure there are some questions here that postdocs would find interesting, even if most aren't. I don't think the community would be big enough to sustain a whole separate site for only advanced questions.