Commentaries, Lam-Rim texts, anthologies of suttas, summaries, and study guides are all organized in a linear fashion.
There have been criticisms of the Nikayas for not being organized in a linear fashion, but linear organization is flat.
Perhaps the order of the unabridged discourses helps us to develop asymmetric thinking that should be appreciated.
From a Google Search on how to develop asymmetric thinking:
"Cultivate creativity and non-linear approaches by broadening inputs (diverse sources, experiences), using visual tools (mind maps), challenging assumptions (Socratic questioning, reverse thinking), embracing experimentation (play, failure as learning), and practicing divergent techniques (brainstorming, freewriting) to generate unique, outsized solutions rather than predictable ones."
According to the Nikayas, monastics and lay people heard whatever discourse was being delivered at the time and over time developed their own understanding by immersion.
I enjoy the way the Nikayas were put together. It’s more challenging and rewarding to read them straight through, ask questions, keep going, and keep returning to the Nikayas.
There are great benefits to reading commentaries, but their importance and usefulness pale in comparison to the Nikayas themselves in terms of authenticity.
A number of Buddhist traditions (sometimes for centuries) have emphasized commentaries at the expense of the Nikayas and then returned to them with great benefit.
The Buddha used many different techniques, questions, experiences, logic, alternative views, visual tools, and beautiful similes to challenge people.
Some important things to consider when reading the suttas, is that they were given as a teaching for the people who were present at that one time, in that one place, with a specific knowledge set. To them they weren’t ancient texts, but cutting edge (mind) tech!
As you mentioned, most of the people in the Buddha’s time only heard a handful of suttas and used them to reflect upon. Our modern thirst for all the information before we commit to a practice might not be the only way.
The organisation of the nikayas depends on the nikaya. The Samyutta is thematic, inviting you to take a deep dive into a specific topic. The Anguttara is more a brief snapshot on different themes. Digha invites you into a story with its myth and characters. Udana and similar KN suttas are to inspire the heart with verse.
If you are just trying to know the variety of what the Buddha did and didn’t teach then starting at the beginning of MN (or MN2 for a slightly easier onboarding), or any prose Nikaya, is a good approach. You can turn on Bhante Sujato’s notes for extra help (hit the N key on desktop). However, if you have been reading suttas for a while then taking a thematic approach, etiher through SN or by using something like http://index.readingfaithfully.org to study a topic from different angles is useful. Asking questions about what and how the Buddha taught on specific topics can be a fruitful approach.
A few years back I asked a related question, which you might find interesting.
