What you're doing is essentially what Zod is designed to avoid. If you tolerate needing a separate build step more than having to define types with Zod's syntax, then it makes sense not to use Zod since it's not made for you.
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).