At some point you should get to the original sources if you want to be an academic, yes.
But the reality is that 99% of the original source material is not relevant, will not be particularly rewarding, and may require a grad-level background to even understand many of the sentences. You want to read Hobbes Leviathan? Guess what, the only parts that matter today are a handful of pages. Same with Kant's Groundwork, Aristotle's Politics, etc.
The famous philosophers are (mostly) famous for their essential ideas, not the full lengths of their books.
For example: the majority of Hobbes' Leviathan is coming up with an entire theory of natural law, dealing with religion, etc. But none of that stuff was particularly noteworthy. The part that's relevant today was his original thoughts on the social contract, which is a very short part.
It's far more important to read textbooks, surveys, etc. to understand the ideas and be able to put each philosopher in context of who they were responding to and who responded to them.
Then if you're an academic or going to write a paper or something, sure go to the actual sources.
I say this having read tons of Plato, Aristotle, Seneca, Kant, Locke, Rawls, etc. I've needed to for my work, but I would never recommend reading them to someone just casually interested in philosophy. That's like telling someone who wants to learn how to program that they should be reading the C compiler source code.
At the same time these primary works are the main examples of philosophy actually being done.
Any summary material can only speak to the results of philosophy, while the primary sources when closely analyzed (like source code!) yield the process. Which will be more helpful if one wants to develop a "personal philosophy"? Working out the chains of reasoning or picking-and-choosing from a menu?
The primary works often just blatantly "state" things as if they were opinions. Then it's up to generates of scholars afterwards to argue about what the actual process of logical argument was.
Historical philosophy books aren't mathematical proofs. They don't necessary demonstrate process at all, it's not philosophy being "done" but just stated. Very often, it's precisely the summary works that situate the thinkers in a context that illuminates the process that led to their conclusions.
Also I don't know what you mean about "developing a personal philosophy"... I'm talking about academic philosophy here.
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.
Original sources are paramount to any real sense of understanding. In my opinion the best approach is reading primary and secondary material in tandem. Understanding ideas is important obviously. But there’s also something to be said about the experience of navigating a difficult text and beginning to grok it, and the process itself is rewarding and worthwhile.
That said if the person asking this question has no interest in being an academic, your advice is horrible. Imagine reading about Plato but never reading a dialogue, never getting a sense of being in a room with Socrates. Imagine reading about the death of God but not a single sentence of Nietzsche’s maddeningly beautiful prose. You need to engage with real philosophy.
But the reality is that 99% of the original source material is not relevant, will not be particularly rewarding, and may require a grad-level background to even understand many of the sentences. You want to read Hobbes Leviathan? Guess what, the only parts that matter today are a handful of pages. Same with Kant's Groundwork, Aristotle's Politics, etc.
The famous philosophers are (mostly) famous for their essential ideas, not the full lengths of their books.
For example: the majority of Hobbes' Leviathan is coming up with an entire theory of natural law, dealing with religion, etc. But none of that stuff was particularly noteworthy. The part that's relevant today was his original thoughts on the social contract, which is a very short part.
It's far more important to read textbooks, surveys, etc. to understand the ideas and be able to put each philosopher in context of who they were responding to and who responded to them.
Then if you're an academic or going to write a paper or something, sure go to the actual sources.
I say this having read tons of Plato, Aristotle, Seneca, Kant, Locke, Rawls, etc. I've needed to for my work, but I would never recommend reading them to someone just casually interested in philosophy. That's like telling someone who wants to learn how to program that they should be reading the C compiler source code.