My sight reading has improved a decent bit over the past few weeks. This is probably just what cracked it open for me, but here goes:
Memorizing a lot of short pieces.
Somehow, just reading through several pieces didn't quite do the trick as much, although it was still useful. But memorizing around 30 pieces of music over the past year, many of which were etudes, somehow forced me to use my music theory knowledge and gave me enough practice sifting through chord patterns in various inversions to recognize many of them by sight. I feel like the aural memory combined with the muscle memory also solidified some of those connections, to the point where when I play another piece which uses traditional harmony, I'm starting to feel almost like I've played the piece before. Of course, I haven't, but it's like a variation on a theme. Also, working through more difficult pieces and memorizing them over the past couple months has somehow made easier pieces seem "easy" in comparison to read at sight.
The one part that I think is surprising is that having those pieces memorized seems to have made a considerable difference.