This is going to sound dead obvious, but I'd honestly recommend some Intro to Philosophy/Logic/Ethics books, e.g. Logic: A Very Short Introduction or The Logic Manual. I know you're not necessarily looking at logic specifically, btw, but it's one of the regular recommendations with philosophy more generally, especially if you branch out to other thinkers.

Also a somewhat common introductory text that I personally didn't care for is Sophie's World, which acts as a narrative historical overview of Western philosophy. If you want that sort of thing you may be just as well with a text like A Little History of Philosophy, which might be available via your library.

Finally, for ethics, you might look to Hornbook Ethics and/or Justice: What's the Right Thing to Do? Besides A Little History of Philosophy, if memory serves all of these I've linked to have at one point or another been required texts for different philosophy and ethics courses depending on where one attends, so in reading these independently you'd basically be getting some intro level course education on the cheap.

Honestly though, I'd say reading the primary texts as you're doing isn't a terrible way to go about it, as that's what many philosophy courses would have you do anyway. What I would recommend alongside that is the same as I'd recommend for any older text then, which is to read up on the historical context to get a better sense of what may have influenced their writing, how they were received by their peers and broader community (if they were noticed at all during their life), and develop your understanding from there.

Sorry if this is a bit much, but I hope it helps! Be sure to check your local libraries for any of these before buying them btw, as they may have hard copies available despite not having digital copies. Also remember that college/university libraries may be an option too, albeit likely more costly if they do offer a library card to borrow materials.