My wife teaches elementary school. Good lessons there on how to approach any vast topic (like writing: start with alphabet, words, sentences, paragraphs but early on get them excited about communicating; add grammar and spelling in small doses along the way; introduce voice, perspective, structure, and related topics as needed, expand on selecting content and improving communication, move on to analogy, symbolism, and much more over the years).
What are you most interested in accomplishing with SONAR? Since people come to SONAR with vastly different prior knowledge, teaching materials that work for one person may not be best for another. For beginners the great challenge is to discover which features and functions are essential and what refinements and details can be put off for later. As you point out, the manuals are written from a narrow perspective of specifics of each tool and module. For SONAR, that takes more than 2,000 pages in the reference manual (installed with SONAR) even without adding material about the fundamentals of recording, arranging, mixing, mastering, synthesizing, harmony, rhythm, notation, etc.
So here is another vote for Groove3 (I am not affiliated with them). The best deal is to get a full year access pass that gives you access to everything they have. Even before purchasing, browse their offerings, and drill down to the list of topics in each course. That is a great way to get a sense of the depth and breadth of what there is to learn. Here are some obvious SONAR starting places among Groove3 courses: SONAR Explained, SONAR Platinum Advanced, Mixing with SONAR. But you can also get a lot out of courses aimed at earlier versions: SONAR 8.5 Explained, SONAR X1 Explained, SONAR X1 Tips and Tricks, and several more.
You might also get a lot from the fundamental courses that are not aimed only at SONAR: MIDI Explained, Music Theory Explained, The Fundamentals of Mixing, Mix School 101, Producing Electronic Music, Studio Secrets, and many others.
People take full multi-year college and graduate courses to learn music production (see Berkeley School of Music for online example, check syllabus and prerequisites for each course), and professionals continue to learn over many decades. So pace yourself and enjoy the journey. Best wishes to you.