I'm about a year into Nix and consider myself a moderate in some areas but something of an expert (by necessity) in others. I think the ideal pattern is to attack it bottom-up and top-down at the same time.
Bottom up, you want to learn the fundamentals of how the Nix language works, and how the basic primitives it offers can be used to build up package definitions, a package manager, and ultimately an entire OS. For this, the Nix Pills are invaluable: https://nixos.org/guides/nix-pills/
Top down, you need goals for some specific things you want to accomplish in the system. For me, this was automated packaging for hundreds of source repos internal to my company, but that's a bit of a crazy case— for a normal person, I'd expect this could be something like "I want to run a webserver with some static assets" or "I want to define and launch a container declaratively", or maybe "I want a reproducible environment to do Python development in, where the reproducibility isn't just tagged versions in a requirements/pyproject file, but also includes the full underlay of everything I'm depending on from the base system.
Whatever the goal is here, you're inevitably going to find your way to override-related tasks, like "okay, I want to upgrade this package" or "I want to add patches to this package" or "I want to change the settings on one of my dependencies", and that's where the extraordinary power of Nix really starts to sink in, when you realise how much can be accomplished with so little, and you try to imagine what accomplishing that under a conventional apt- or dnf-type system would look like.