For me the biggest benefit is the ease of applying patches. For example in Nix I can easily take a patch that is either unreleased, or that I wrote myself, and apply it to my systems immediately. I don't need to wait for it to be released upstream then packaged in my distro. This allows me to fix problems and get new features quickly without needing to mess with my system in any other way (no packages in other directories that need to be cleaned up, no extra steps after updates to remember, no cases where some packages are using different versions and no breaking due to library ABI breaks).
Another benefit that you are pointing at is changing build flags. Often times I want to enable an optional feature that my distro doesn't enable by default.
Lastly building packages with different micro-architecture optimizations can be beneficial. I don't do this often but occasionally if I want to run some compute-heavy work it can be nice to get a small performance boost.
Sounds great. I think it is super valuable to have an RSS feed so that people can subscribe in all sorts of ways. Having ActivityPub is also nice.