You seem to want something specific, so I'm going to take a different approach than the others and offer a single solution to your constraints.
One word: Wireshark.
The tool is free and is in the top 3 tools network pros use regularly. It is possibly the best tool for packet analysis. I taught networking and protocols as a Department Head for a technical school, and I continue to learn more from Wireshark than I could from any other source. It helps you understand both the technical details of protocols as well and the overall concepts.
The other reason to focus on Wireshark as the omnibus approach to your question is the books and training available. Yes, there is a certification, but that need not be the focus. The training will walk you through the practical aspects of protocols without delving too deeply into the theory, which seems to be what you are hoping for.
Wireshark also comes with collections of packet captures for you to dissect and analyze, which can enhance your learning in both packet concepts as well as security applications.
Using this targeted approach, I truly believe that you could:
- avoid 'all the little tools' while gaining a better understanding of the tools you then choose to pick up
- gain a good foundation in applied protocol theory
- gain familiarity and experience with packet analysis
- accomplish something worthwhile in 25 days
Augmenting the Study Plan
As you run into questions or when you want to know more about what you are looking at in a packet dump, start working through @D.W. 's list.
Study Plan Advice
My other advice is to pursue the areas that interest you the most because 'security' is such a wide field. You seem to focus on packet-level security, so do that, and build knowledge there than can be transferred to other areas as you become interested.
I hope this triggers some ideas for you.