I haven't read any of these books (although I don't know what "Greek Literary Papyri 1 Poetry" contains), but anyway...
I haven't read Crito, but I agree that at least the Apology is comparatively easy. In my experience, the difficulty of Plato varies a lot from the text to text, so not everything from Plato is easy.
Another "beginner's" text is Xenophon's Anabasis. But I have never really seen an easy Attic text, the way I think New Testament Greek can be (relatively) easy.
I agree that Euripides is a lot easier than Sophocles or Aeschylus. But it's still rather difficult.
I don't think Sappho is at all that difficult. It looks a bit different at first sight, but you get soon past that by just learning the basic phonetic correspondences with Attic. This is oversimplified, but it works to a certain point. After that, Sappho isn't very difficult. And the corpus is quite small – If you read only the well preserved poems, and not every one line fragment, that's only a few pages. If you're interested at all about Sappho, I think you should give it a go from early on; even if it's a struggle, you can read it quickly, because there's really not that much. Of course, you should have read some Attic poetry or even better, Homer, first. Aoidoi.org has a short but nice introduction to Lesbian Aeolic