Design patterns for composability with traits (i.e. typeclasses)?

Perhaps so. But it will be adding a lot of complex boilerplate that wouldn't be necessary with my proposal in order to unconflate the type checking from dependence on the order of adding being consistent with the order of enumeration.

It seems it will make the code very difficult to understand.

It seems my proposal removes that for a K.I.S.S. result. But we'll have to study code examples of both design patterns to make sure.

And I do intuitively believe that the degrees-of-freedom of composability will suffer in your design patterns, because it is always working against itself to unconflate typing from data structure. Afaics, it will require lifting to higher abstractions for each new degree-of-freedom desired, adding more complex boilerplate but never being entirely unconflated for all degrees-of-freedom