Flash cards are not great here since you loose context, you want to study both the vertical harmonic structure of chords and how that evolves through time in the horizontal structure. Just download a bunch of sheet music and print it out or buy a notebook/pad of staff paper and write it out.
That's exactly why I staled and wrote here, the flashcard system felt off to me.
Thank you for your detailed feedback, it's really helpful. It's good to read that there is no secret sauce for theory either.
But I'll restate what I said elsewhere, I think ear training is much more important once you get past the basics. In that case, I'd learn the Nashville Number System (https://en.wikipedia.org/wiki/Nashville_Number_System).
Part of my education, in commercial ear training class, we had to write number charts and get it right the first time through. It sounds hard, but with practice it's really quick.
Sight reading is more than the note names, it is also the action of moving your fingers and not just finding the note on the keyboard but keeping your hands in the right position so your others fingers can do what they need to do. So you set your metronome nice and slow and play through your scales while singing the notes and get ear training thrown in for free. Flash cards will just add in an extra step here, you still have to learn those finger movements, sight reading and ear training. Just remember to speed up that metronome when things get easy. The Theory side is much the same.
The single best thing you can do is make practice a part of your daily routine, an hour a day every day. If you are doing that or close to doing that then you need to talk to your teacher about the lack of progress and possibly find a new teacher.
I took 9 years of piano lessons, but I was very young (I quit when I was 12, 25+ years ago), so I have a vague memory that there's a "right" way to do things, but no idea where to go to re-learn it.
I quit as soon as my parents would let me because I thought sight reading was supposed to be as easy as reading written words, and if I hadn't been able to master what I thought of as the most basic skill in 9 years, I must be hopeless.
Now I understand that sight reading is actually a much more advanced skill, so I probably wasn't hopeless after all.