I have tried to achieve a multilevel sidenav and I found this that meets my requirements:
angular-material-multilevel-menu
Demo - Note the accordion type
Unfortunately this is created for AngularJS (1.0?), which seems to not work in Angular 6.
My questions are:
I don't know if you are still looking for angular-material-multilevel-menu but I have found one made by ShankyTiwari. Here is the link for GitHub and the link for the demo.
Very easy to use and to implement. For example, I implemented it in a sidebar because it does not exist with Angular material. If not an alternative would be PrimeNG as @Dino said.