I agree that spaced repetition is often done poorly and needs to be customized based on performance.
Just putting this here because you might find it interesting: I spent several years building a fully-individualized spaced repetition system that not only
1) works in a hierarchical knowledge domain (where repetitions on advanced topics "trickle down" to update the repetition schedules of simpler topics that are implicitly practiced), but also
2) minimizes the number of reviews by choosing reviews whose implicit repetitions "knock out" other due reviews (like dominos), and
3) calibrates the spaced repetition process to each individual student on each individual topic (student performance and topic difficulty are competing factors).
Just putting this here because you might find it interesting: I spent several years building a fully-individualized spaced repetition system that not only
1) works in a hierarchical knowledge domain (where repetitions on advanced topics "trickle down" to update the repetition schedules of simpler topics that are implicitly practiced), but also
2) minimizes the number of reviews by choosing reviews whose implicit repetitions "knock out" other due reviews (like dominos), and
3) calibrates the spaced repetition process to each individual student on each individual topic (student performance and topic difficulty are competing factors).
You can read more about it here if interested: https://www.justinmath.com/individualized-spaced-repetition-...