To me the build step is a good thing. It's a simple script in npm, and it means I only keep what I need (the JSON Schema, which I don't need at dev time) in runtime and whatever package generates those schemas out of TS types can remain as a dev dependency.

zod can't be a dev only dependency, and you have to deal with breaking changes and maybe switching to a completely different library in a few years (joi, with a syntax very similar to zod's, was very popular a while ago too).