That seems like a big over simplification. Speaking for myself as someone who was very much an above-average student, I often thought I was following along fine

That seems like a big over simplification. Speaking for myself as someone who was very much an above-average student, I often thought I was following along fine in the lectures but then when I would go to solve a homework problem or answer a test question, only then would I realize I skipped over some important parts in my understanding. So even the best intentioned students are likely not in a position to diagnose their own understanding of the material, that's what tests and quizzes and homework are for. And that's not even to mention, any of the students that know they aren't following along, but don't really care and would rather zone out/sit on their phone the whole lecture than get 1:1 help. Do you really expect most students will seek help for themselves? Perhaps you could modify your plan and say that students who got lower than a B+ or something on a recent quiz are required to attend the breakout group to get help from the teacher, it's not up to them to self-select. But then you're back to... the status quo, that "breakout group" is just the regular classroom. You have one class for the advanced students to go through material quicker, and another class for everyone else. That's already the way many schools are structured.