Custom Variables in custom FormType - symfony

Question

Got some question about passing custom variables to a custom FormType
I have a custom FormType named KontoType:
I pass some custom variables to it and this works like as expected, if i override the method buildForm and dump the passed $options array, the mandant exists and is a entity.
But how the heck can i now pass this custom variable to the function getChoices() which loads the choices based on the mandant in this custom FormType?
Even if i did reset the $options in the override buildForm function like $options['choices'] = $this->getChoices($options['mandant']) the select box is empty if i render this form.
<?php
namespace App\Form\Type;
use App\Entity\Core\Finanzen\Konto;
use App\Entity\Core\Organisation\Mandant;
use App\Services\LocaleService;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Translation\TranslatorInterface;
class KontoType extends AbstractType
{
/**
* #var ObjectManager
*/
private $manager;
/**
* #var TranslatorInterface
*/
private $translator;
/**
* #var LocaleService
*/
private $localeService;
public function __construct(ObjectManager $manager, TranslatorInterface $translator, LocaleService $localeService)
{
$this->manager = $manager;
$this->translator = $translator;
$this->localeService = $localeService;
}
private function getChoices(Mandant $mandant=null)
{
return $this->manager->getRepository(Konto::class)->findBuchbar(true, $mandant);
}
public function configureOptions(OptionsResolver $resolver) {
$resolver->setDefaults(array(
'label' => 'konto.name',
'help' => 'konto.help',
'choices' => null,
'attr' => array(
'class' => 'bs-select',
'aria-hidden' => 'true',
'ref' => 'input',
'multiple' => false,
'tabindex' => 1,
'data-live-search' => true,
'data-size' => 6
),
'choice_label' => function ($choiceValue, $key, $value) {
return $choiceValue->getKonto()." ".$this->localeService->doTranslate($choiceValue);
},
'choice_value' => function(Konto $konto = null) {
return $konto ? $konto->getId() : '' ;
},
'required' => true,
'multiple' => false,
'empty_data' => null,
'label_attr' => array(
'class' => 'control-label'
),
'placeholder' => 'message.bitte wählen',
'mandant' => null
));
}
public function buildForm(FormBuilderInterface $builder, array $options)
{
$options['choices'] = $this->getChoices($options['mandant']);
parent::buildForm($builder, $options); // TODO: Change the autogenerated stub
}
public function getParent() {
return ChoiceType::class;
}
}