Aquinas himself is pretty readable. Having read a few intros, like Feser's, to Aristotelian-Thomistic philosophy is necessary, but from there, you can read the Summa directly, starting wherever you like.

I do wish I had done that. I have read a lot of commentary on Aquinas and comparatively little Aquinas. I'm finally venturing deeper because it's hard to justify taking exegetical positions when you haven't read the original!