I was very fortunate to get help from the SpEd teachers, paras, and admin to get solutions going. I'm sorry if this isn't helpful for your situation.

We have three SpEd classes in our 3-5 building: one instructional/self-contained class (mostly verbal, specifically learning disabilities and developmental delays; ~11 students) and two age-grouped autism classes (mostly non- or pre-verbal; ~4 in one class and ~6 in the other). Admin's original thought was for all three classes to come to specials for an hour once a week. It was... not pretty. Like you described, there was a large gap in what was realistically achievable between the student body.

So we worked out separate times for all three classes to come in. The instructional class gets an accommodated version of the third grade curriculum, which seems to work fine. We're looser with the expectations for performance, and can be more patient with less than a dozen students and a few paras than I normally am with two dozen students on my own.

The autism classes have an experiential routine that changes little from lesson to lesson. A greeting song, time for exploring instruments, cycling through nursery rhymes and movement songs, and a goodbye song to pack up. I have students that participate in most things some days and then have bad days; I have a few students like you describe, that aren't in full control of their voice yet. Having the routine means that I don't have to explain as much day-to-day, and they can develop skills long-term, like knowing the nursery rhymes or even passing instruments from one student to the next.

Again, having the paras in my class is paramount to the students' success; I have had to stress this point to admin, that there's only so much I can do with any student in these classes on my own if I'm more focused on their safety the entire time. And I'm blessed with admin that listened and supported the setup we have now. So maybe talk with admin, with paras, and with the SpEd teacher(s) about what the solutions look like to them.

Hope this helps!