Unfortunately I'm in the rather unhelpful position (which I suspect is the position most philosophers also find themselves in, hence producing the problem you've so astutely pointed out) of not really remembering how I got into philosophy. I took a few college courses, read the textbooks, then started Googling the names of philosophers that seemed interesting and reading primary or secondary sources about their ideas. So all I can really offer is the sort of textbooks I began with, because I do think I found their format uniquely helpful for getting me into the thick of interesting things quickly: namely, primary source anthology textbooks!!! These are great :D I love them to pieces and always intend to read more.
Basically, try to find books that are composed entirely of the juicy sections of primary sources on a given subject from all sorts of different points of view, preferably with some footnotes and introductions to give proper context. Not handbooks or something where it's content by many authors, but written for the occasion — stuff made of primary sources. Things like this[0], although I haven't read that particular specimen since most of the ones I read to get into philosophy were bespoke ones made by my professors. Also, the Stanford Encyclopedia of Philosophy is indispensable for getting a real sense for the philosophical debate about any given topic — the arguments back and forth in all their minutae. Also have Wikipedia handy for definitions of technical jargon (always read the "criticism" or "responses" sections! and also remember Wikipedia is often incomplete).
In my opinion then the general point is that to become familiar with, and even good at thinking and arguing about, even rather deep philosophy, you don't necessarily need to sit down and munch through a dense and possibly dull primary source or didactic textbook from cover to cover unless you want to. Engaging with primary sources is good, but doing so when and where it seems interesting, on your own terms, in bite sized chunks, with secondary sources to supplement and broaden your horizons, is more likely to avoid burnout and help you digest things, so it's better in the end.
Also, I highly recommend Philosophical Investigations, it's very good and reasonably accessible from what I've read of it.
Basically, try to find books that are composed entirely of the juicy sections of primary sources on a given subject from all sorts of different points of view, preferably with some footnotes and introductions to give proper context. Not handbooks or something where it's content by many authors, but written for the occasion — stuff made of primary sources. Things like this[0], although I haven't read that particular specimen since most of the ones I read to get into philosophy were bespoke ones made by my professors. Also, the Stanford Encyclopedia of Philosophy is indispensable for getting a real sense for the philosophical debate about any given topic — the arguments back and forth in all their minutae. Also have Wikipedia handy for definitions of technical jargon (always read the "criticism" or "responses" sections! and also remember Wikipedia is often incomplete).
In my opinion then the general point is that to become familiar with, and even good at thinking and arguing about, even rather deep philosophy, you don't necessarily need to sit down and munch through a dense and possibly dull primary source or didactic textbook from cover to cover unless you want to. Engaging with primary sources is good, but doing so when and where it seems interesting, on your own terms, in bite sized chunks, with secondary sources to supplement and broaden your horizons, is more likely to avoid burnout and help you digest things, so it's better in the end.
Also, I highly recommend Philosophical Investigations, it's very good and reasonably accessible from what I've read of it.
[0]: https://www.amazon.com/Analytic-Philosophy-Anthology-P-Marti...