I would recommend Bazzite or Aurora/ Bluefin. Bluefin is Gnome, and Aurora is KDE.
Both Bazzite and Bluefin are very similar. Bazzite is gaming focused, and the other one is more general purpose, but you can use them interchangeably and also rebase from one to the other.
They are the poster childs of the uBlue project, which uses, modifies and redistributes Fedora Silverblue images.
They both are part of the Fedora Atomic family, which makes them nearly indestructible, convenient and secure.
They focus a lot on containerised workflows, e.g. Distrobox, Flatpak, Homebrew, and, as you mentioned, Nix. They all come pre-installed, and if they don’t work ootb (e.g. Nix), they are just one ujust command away.
I have used both over the past year and I couldn’t be more happy. Give them a try!