Yeah, 30 is too much. I think 16 is an upper limit and even then you need to break it into groups of ~4 to actually have small group teaching. Of course the universities never have money for that many assistant teachers for the bigger courses. The point is that the group needs to be small enough so that people can get to know each other. Then you can have a relaxed atmosphere and then the real discussion can start.

I think we are on the same page about your last paragraph. You are right, you face a problem of a same kind that I wrote about when learning in a group. Someone seems to know the material well, they are confident and maybe even charismatic. They go through their solution and everyone just nods and agrees.

The trick is to get everyone tell their solution (maybe not for the same problem) and have an atmosphere where people feel comfortable correcting and getting corrected. That is a key responsibility of a small group teacher.

The "peril" in self-study is a bit different. You may find your enlightenment but how do you know that your mental model is correct?

Right, I'd still need a bit of external test after isolated work in many case. But sometimes small abstract concepts will unlock and you just know. Aha moment.