Perhaps surprisingly, I believe the core deliverables and thinking of a PMM are easiest to improve. These are strategy frameworks, effective messaging and positioning, etc. You’ll learn most by doing, and get better with time, especially with retros to look back on previous launches and strategies.
The most important and hardest skill to improve is influencing stakeholders outside of marketing. This skill will only grow importance the higher you go in the organization.
I’ve seen excellent marketers and otherwise great leaders struggle to influence the technical orgs which, depending on company culture, may hold more sway in a tech-led organization. Delighting internal stakeholders and delighting customers are not always perfectly aligned. For example, I’ve seen a situation where a PM wanted us to announce a product pre-maturely, so they could list it for their performance review.
The best marketing leaders I’ve had the pleasure of working with are able and willing to challenge internal thinking to always put the user first.