How do I create a growth plan with my manager/mentor and what should it include?

I recently joined a startup (just finished my third week about to start my fourth) and I set up a one-on-one last week with my manager to hopefully create a mentor-mentee relationship. That went well and now I’m looking to create a growth plan and a way to measure my performance. We’ll be checking in on a weekly basis so I can get a sense of my performance and how I am growing as a engineer.

Given that it is a Series B startup with ~25 people, our goals will be shifting a lot which makes long-term growth plans hard to create. For context, since I joined, my 60 day and 90 day projects have shifted somewhat. So how do I create a growth plan that is flexible and what should it include (Concrete goals vs. more behavior orientated goals)?? Should the growth plan be used separately from judging my performance or would it be the measuring stick for performance?

Would really appreciate some insight on this.