How would you establish Product Marketing in a 500-employee company from within the product team that only focuses on product development without influence over marketing and sales?

In this scenario, I think it can be helpful to explore initiating a product marketing function from other disciplines in addition to Product. Since product management and product marketing are so closely related, if not originally part of the same function, carving out a PMM function from within product is of course the most natural extension—i.e., there are likely already some product managers or product ops folks that spend more time looking at external/business-outcomes, instrumenting ways to measure adoption, sources driving desired outcomes, etc. But there may also be folks in other functions for which transitioning to a formal PMM role would be a natural transition—they could be AEs that have been building their own enablement materials and refined the product's positioning and messaging through deep insight from customer calls. Or similarly, demand gen/growth marketers that have been crafting their own product messaging and landing pages for various experiments and have gotten close to the product and audience as a result.