So... The "big idea" would be "a writing system that fully represents a language."

Everything else can develop gradually. But... gradually doesn't necessarily mean generations. It can be one person expirementing, working on the project and until completion.

Think of it as a "master work" or a PhD. Big, but not beyond the right person's ability to complete over a decade.

Think Newton, Galileo or Tolkien. They didn't just "have an idea" that fully worked. But... they worked on their ideas for years, got a lot done themselves, and had fruitful projects.

People are people. What is possible in one century is possible in another, unless prerequisites are not present.

In the case of language... the centuries of development mostly contribute the ambition itself.

The inventor may not be doing some great project. They might be just inventing something small to help keep track of sales... or they're just inventing a gambling game that uses symbols.