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.

I've worked through this very scenario on a few occasions. I think as startups mature and grow there is often an understandable resistance to 'letting go' of all the parts of the business that the founders had to manage initially. Sometimes they have areas of particular interest and those are often the hardest (i.e engineering, product development, some even are 'aspiring' marketers!) I think and easy method is to open yourself to collaboration with them if at all possible. They hired product marketing for a reason right so apply that to work with them on evolving their earlier messaging. Are there other areas of the business that have needed change/evolving as the business grew and matured? Use that as an example that ALL areas need to evolve including messaging. Bring in customer feedback and 1st hand data to support a need for a change..thats very hard to argue with. Hopefully get some senior level support and even propose an isolated test to help prove (or disprove) the opportunity. I did that once where we could isolate a particular market quiet well and run some different messaging without impacting our key markets/avenues and it gave us enough info to be able to expand and eventually role out an entirely new messaging hierarchy.

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.