I feel similarly. I've performed the Chopin/Liszt etudes and Bach's sinfonias as a kid (which I guess translate to intermediate classical piano skill) but would struggle to sight read even the two-part inventions at 1/2 or even 1/4 speed.
I'd be quite keen to use some method to upgrade my sight reading to where I find learning new music rewarding, as long as it's known to produce results.
Currently I can learn a Chopin Nocturne or Mazurka much faster by ear (listening to it to learn the rhythm/melody/harmony) to recreate it roughly and watching someone play it to get the more exact voicing (with the sheet music as a reference mostly).