Django QuerySet function that will return up to `max_length` objects from the `selected` queryset, falling back to the `fallback` queryset to make up the numbers.

queryset.py

def get_selected_or_fallback(*, selected: QuerySet, fallback: QuerySet, max_length: int) -> list:
    """
    Return up to `max_length` objects from the `selected` queryset,
    falling back to the `fallback` queryset to make up the numbers.
    """
    selected = list(selected[:max_length])
    fallback = list(fallback.exclude(id__in=[obj.id for obj in selected])[:max_length])
    return (selected + fallback)[:max_length]