I'm a PMM who typically executes campaigns, develops sales collateral, manages product launches, etc. How do I gain experience developing high-level company messaging, category creation/development, etc?

Just start working on it! There are so many opportunities to do this, worse case no one notices best case you start to get pulled into really strategic conversations.

A great example of this is AI. Every B2B software comapny needs an AI narrative right now. If you're to help tell that story and just go out and start doing it without being asked, good things will happen.

I will say it's best to think globally but start locally. Focus the AI story on your products and your industry or segment. Don't try and rewrite the company narrative as an IC PMM or even Director. It's too big and will be frowned on.