Wagtail: How to filter FieldPanel dynamically for ForeignKey values?

I have this Page with a ForeignKey districtobject:

class DistrictPage(Page):

    districtobject = models.ForeignKey(DistrictTranslated, on_delete=models.SET_NULL, null=True, blank=True)
    streamfield  = StreamField(BasicStreamBlock, null=True, blank=True, use_json_field=True)

    content_panels = Page.content_panels + [
        #InfoPanel(),
        FieldPanel('districtobject', classname="full"),
        FieldPanel('streamfield'),

    ]

In the wagtail admin interface the dropdown menu have to much values for districtobject. I want to implement a dynamic filter, which shows only a few entries based on the parent Page object.

How/where can I implement my own queryset for this dropdown?

I found the following solution:

class DistrictPageForm(WagtailAdminPageForm):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self.instance and self.instance.get_parent():
            parent_page = self.instance.get_parent().specific
        else:
            parent_page = kwargs.get('parent_page', None)

        if parent_page:
            self.fields['districtobject'].queryset = DistrictTranslated.objects.filter(city_id=parent_page.cityobject.id)

class DistrictPage(Page, CityVariables):

    districtobject = models.ForeignKey(DistrictTranslated, on_delete=models.SET_NULL, null=True, blank=True)
    streamfield  = StreamField(BasicStreamBlock, null=True, blank=True, use_json_field=True)

    content_panels = Page.content_panels + [
        FieldPanel('districtobject', classname="full"),
        FieldPanel('streamfield'),
    ]

This works fine for viewing, creating, editing and deleting in my case. Not sure if this is the best practice doing this.