django_filter custom Filter for jsonfield howto populate filter dropdown?

Question

I am working on a django_filter for data stored in a jsonfield (no, for now I want to keep it this way).

    payload = models.JSONField()

I developed a custom filter to emulate the behavior of athe filter for normal fields. I've got it almost working like the "exact"-behavior of a normal field. Only thing is the Choice-Dropdown is empty.

class EventFilter(django_filters.FilterSet):

    class Meta:
        model = Message
        fields = ('event_type', 'origin')

    event_type = django_filters.ChoiceFilter(
        method='type_filter',
        label='type'
    )

    origin = django_filters.CharFilter(
        method='origin_filter',
        label='origin'
    )

    def event_type_filter(self, queryset, name, value):
        return queryset.filter(payload__event_type__exact=value)

    def origin_filter(self, queryset, name, value):
        return queryset.filter(payload__origin__exact=value)

Any ideas how to make it populate the ChoiceFilter?

Update 24.11.22: Improvements so far (actual filtering does not work but values show up):

I used this filter code:

event_type = django_filters.ModelMultipleChoiceFilter(
    label='Event Type',
    lookup_expr='exact',
    field_name='payload__event_type',
    queryset=Message.objects.all().values_list('payload__event_type', flat=True).distinct(),
)

I thought the distinct() would clear duplicate values but probably I am using it wrong... Ideas?