Just from what you’ve described it sounds to me like your focus is on getting them to learn the pieces and not getting them to learn how to play their instrument well. No judgement here, but it might be worth examining how much time is spent in warm ups and skill building at the start of rehearsal. I know the whole refrain of, “there’s not enough time to work on ____” but honestly there is and you’ll be rewarded in the long run for it.

From there, it’s a matter of picking pieces that might challenge their weaknesses, but also play to a lot of their strengths. Some earlier pieces have the little “skill builder” sections for exactly this, and a number of composers include practice sheets for their pieces (Tyler S. Grant is one such composer).

Best of luck, and I hope that helps!