Is it possible to transition to senior product marketing roles from a digital marketing or brand management role? What should be some of the key aspects to highlight in the resume to stand out?

This is 100% possible. I made the exact move from Digital Marketing to Product Marketing just a year ago. To make that switch, I would highlight a few areas, and you should look to build this experience if you don't have it yet.

1. Prove you're customer insights-driven. If you clearly demonstrate that you've somehow researched customers and implemented changes based on the insights gathered, you'll be off to a great start. Don't just describe an A/B test, though. Qualitative insights can be gold for this purpose. Even viewing session recordings on your site to reveal user path issues could count toward this. Perfection would be conducting interviews about the site and whether it helps visitors accomplish their desired tasks.
2. Demonstrate your ability to prioritize market opportunities. Detail an accomplishment or project that describes a time you identified an opportunity (ideally an external market), weighed it against alternatives, and put together a plan to pursue the opportunity.
3. You'll likely need to demonstrate some proficiency in the company's product or familiarity with the market they're in. You will probably be able to relate your marketing capability without too much issue, but the other two big rocks of Product Marketing knowledge need to be addressed (Market Knowledge and Product Knowledge).