Problems with django_filters.Filterset

[Lookszz]

I have a filter which can be used to filter the products on brand(s), which looks like this:

class SortFilter(django_filters.FilterSet):
    ORDER_BY_CHOICES = (
        ('-discount_sort', 'Hoogste korting'),
        ('-new_price', 'Hoogste prijs'),
        ('new_price', 'Laagste prijs'),
    )

    order_by = django_filters.ChoiceFilter(label='Sorteer op', choices=ORDER_BY_CHOICES, method='filter_by_order',
                                           empty_label=None)
    brand = django_filters.ModelMultipleChoiceFilter(queryset=Product.objects
                                                     .order_by('brand')
                                                     .filter(categorie='eiwitten')
                                                     .values_list('brand', flat=True).distinct()
                                                     , widget=forms.CheckboxSelectMultiple)
    q = django_filters.CharFilter(method='multiple_field_search', widget=TextInput(attrs={'placeholder': 'Zoek..'}))

    class Meta:
        model = Product
        fields = ['brand', 'q']

    def filter_by_order(self, queryset, name, value):
        return queryset.order_by(value)

    def multiple_field_search(self, queryset, name, value):
        return queryset.filter(
            Q(title__icontains=value) | Q(categorie__icontains=value) | Q(brand__icontains=value)
        )

As you can see, the brand filter returns a flat list of all possible in the categorie 'eiwitten'. Is there a way to make the queryset in brand a variable which I can input from my view function? This way I can use the filter for every page. Resulting in a unique brand list depending on the categorie. My view function for the 'eiwitten' looks as follows:

def eiwit(request):
    #print(request.GET.getlist('brand'))
    # filter alleen eiwitproducten
    eiwit_list = ['eiwitten']
    eiwit_filter = Q()
    for item in eiwit_list:
        eiwit_filter = eiwit_filter | Q(categorie=item)

    brand_list = request.GET.getlist('brand')
    brand_filter = Q()
    for b in brand_list:
        brand_filter = brand_filter | Q(brand=b)

    products = models.Product.objects.filter(eiwit_filter).filter(brand_filter)
    product_amount = len(products)

    # sorteer filter
    # HERE I WOULD LIKE TO INPUT A VARIABLE (cat for instance) WHICH CONTAINS THE CATEGORIE BELONGING TO THE VIEW
    filtered = SortFilter(
        request.GET,
        queryset=products
    )

    # paginator
    paginator = Paginator(filtered.qs, 12)
    page = request.GET.get('page')
    try:
        response = paginator.page(page)
    except PageNotAnInteger:
        response = paginator.page(1)
    except EmptyPage:
        response = paginator.page(paginator.num_pages)
    product_front_end = {
        'final_products': response,
        'filter': filtered,
        'count': product_amount,
    }
    return render(request, 'producten/eiwit.html', product_front_end)

The actual filter: