I agree with your assessment of the results.
However, if you look at it on a graph and compare it with a drawing of an ideal graph. You will see that you are really close. This is what I would call being within 5% of the goal.
It's good enough to use and you can go ahead and take photographs.
Develop the next one longer, for sure. But even if you just do what you have here...
Variable Contrast paper will be able to easily work with the negatives you have just made.