I grepped for "nix" in my reader and added some extras:

• https://blog.nixbuild.net/index.xml
• https://blog.thalheim.io/posts/index.xml
• https://devenv.sh/feed_rss_created.xml
• https://discourse.nixos.org/c/announcements/8.rss
• https://discourse.nixos.org/c/dev/rfc-steering-committee/33.rss
• https://discourse.nixos.org/c/links/12.rss
• https://discourse.nixos.org/c/meta/nixos-foundation/47.rss
• https://discourse.nixos.org/t/breaking-changes-announcement-for-unstable/17574.rss
• https://domenkozar.com/feed/index.xml
• https://feeds.transistor.fm/full-time-nix
• https://flokli.de/posts/index.rss
• https://flox.dev/rss/feed.xml
• https://garnix.io/feed.xml
• https://guix.gnu.org/feeds/blog.atom
• https://lix.systems/blog/index.xml
• https://mobile.nixos.org/index.xml
• https://nixcademy.com/feed.xml
• https://nixos.org/blog/announcements-rss.xml
• https://nixos.org/blog/newsletters-rss.xml
• https://nixpkgs.news/rss.xml
• https://numtide.com/blog/rss/
• https://planet.nixos.org/atom.xml
• https://reproducible-builds.org/feed.xml
• https://trofi.github.io/feed/atom.xml
• https://weekly.nixos.org/feeds/all.atom.xml
• https://weekly.nixos.org/feeds/all.rss.xml
• https://www.tweag.io/rss-nix.xml

I haven't used them, but Discourse seems to offer RSS feeds. The NixOS Discourse gets most news worth hearing posted.

https://meta.discourse.org/t/finding-discourse-rss-feeds/264134

I would probably follow the daily or weekly top topics and the announcements topic.

Not the type of feed you're looking for, but an RSS feed nonetheless: Full Time Nix.

Other than that, I'm also subscribed to the determinate systems blog.