From my experience, it's usually product that is doing much of this prioritization, rather than PMM. What I've seen that PMM can do to help is two-fold:
1. Themes. Product knows the product so well, it's very easy for them to get lost in the nuances and intricacies of how different features might or might not be easy to implement in a particular order. But this is a constraint, rather than the critical prioritization framework, which should be market-driven. So help product create a market-driven prioritization framework, by thinking about the important themes that will reinforce and improve you company's positioning over time. Be a consistent advocate for putting priorities into those themes, and scrutinizing especially skeptically those items that don't have a theme at all.
2. Take the view of the voiceless customer. Your question asks about customers and prospects, but not future prospects in future segments. Make sure you're think about them, too, because likely no one else in the business is. It's very easy in B2B SaaS to let your feature backlog get wrapped around the finger of your most important customers. But unless these customers are also representative of the customers you need to grow to your next milestones, they could be leading you astray. So don't forget about the customers who don't have a voice at all!