Question: "For being promoted from SWE I to SWE II, how do I take the behaviors my company has associated with each role (below) and make that more concrete for a growth plan, taking into account the changing & flexible timelines startups have?"
For context, I already have weekly one-on-ones with my manager (who is new at being a manager & is also my mentor), and a growth plan (that I created with him) that roughly outlines (meets most expectations, meets expectations and exceeds expectations for my role). Additionally, keep in mind I work at a startup w/ <30 people so highly specific concrete goals set on a particular date can change in 2-3 weeks as priorities change. Also, my company has defined a series of behaviors as to what each SWE level should be able to accomplish. Here it is.
Software Engineer I (<1 year - 2 years)
Software Engineer II (2-6Years+)
how do I take the behaviors my company has associated with each role (below) and make that more concrete for a growth plan
First of all, I want to stress that one of the reasons people want to work at a startup is the ability to own large scopes no matter what ur current level is. What really helped me grow faster compared to lots of my peers is this mindset
show up, own the hardest problem from the get go. It's ok to fail. As a matter of fact, you should frequently fail. If you are not, it means you are not challenging yourself enough.
Now to your specific question. I think on a tactical front, the rubric you shared already made everything pretty clear
The transition from SWE I -> SWE II (E3 -> E4) is very straightforward. It's basically you doing more stuff better and faster.
Leverage your manager to make sure you are on track and you should get there relatively fast.