Yeah, it feels like the more conventional way would be doing the template-filling as a build step, so it's Nix just for the overall structure managing a fixed build graph, and then a per-page Python run to actually fill templates and generate the snippets/pages.

However, if you did want it to be nix all the way down and get more of that juicy dependency granularity, it could be more of a model where content and templates are transformed upfront into Nix source, which is then transformed and combined during evaluation. Obviously this is fairly slow in an IFD world where all that pre-work has to be serialized, but it would be interesting to experiment with what it looks like in a dynamic derivations context.