First off congrats on the launch!
> We spent weeks talking to 100s of engineers and discovered that their career bottleneck was not coding ability, but all the other _stuff_ that is essential for software engineering.
At this point I've managed hundreds of software engineers and my assessment of the situation is in line with yours. So much so that I actually have a deck prepared for explaining this exact point to engineers.
What I've found is not that engineers need any one piece of advice. That would be different for different people (for the most part), it's that they need a framework to understand all the skills which are important to being an engineer and our industry has woefully underserved them when it comes to explaining things.