The goal of the OP was stated at the bottom of the post as developing a personal philosophy. You can take it up with them, but I interpreted it as coming up with a reasoned worldview for oneself through contemplating existing philosophies.

I actually agree that philosophical works are not mathematical proofs. But thought processes do not close over logic, they close over natural language. The point of reading primary texts is to interpret the text, to reverse out the thought process of what is being said through close reading and exposing hidden assumptions (i.e. hermeneutics). This isn't a strictly logical process, but it's common to all natural-language argumentation, including philosophy and law.

With that in mind, if Kant, Aristotle, Seneca and Rawls count as primary works – then what you're saying is false. They didn't state things in a vacuum; they all developed their views in a context, some of it from some base observations, much of it responding to interpretations of other points of view.

You have a point in that primary sources are not sufficient on their own, and scholars use their experience and erudition to understand these observations and fill in the context, particularly when the writing is otherwise more sparse. But neither would introductory texts be sufficient since they hide a lot of the tacit knowledge that could allow one to reason the way the author of a text reasoned when they produced their philosophical viewpoint.