Our founders have created our product messaging based on their ideas. How do we get them to give product marketing more control over messaging?

Your founders will have a stronger org when they eventually trust product marketing with more control over messaging. If you can, start with research. Talk to prospects and customers and show how they respond to different approaches. And test and learn. If you have an existing audience base, you can try different approaches to a/b testing messaging through various marketing channels (i.e., digital advertising, email, website, etc.). Identifying quick ways to validate and challenge hypotheses on messaging will help shape a perspective to share with your founders when you don’t have a huge budget or time for indepth research before going to market. And then create a moment to thoughtfully present your thinking. Send a pre-read, facilitate a discussion. Show your expertise in the customer and a messaging maven.

Last thing I’d add - your job is to help the org make better decisions. So you don’t have to control it, you just have to make it better.

Don't frame it as a way for "product marketing to get more control over the messaging". Frame it as a way to bring the voice of the customer into your messaging. Product Marketing should be the voice of the customer within the organization and the VoC should deeply influence the messaging. It's not easy for founders to release control, so they may still want to be involved and that is NOT a bad thing. Use their insights, along with the voice of the customer, to develop messaging and measure its success. Be transparent with what is working and what isn't.