I'm still a bit of a noob but I say you go back to default settings and create another model which has a OnetoOne relationship with the User model that way you can add additional fields about a user account & etc and not break things on the backend.
from django.contrib.auth.models import User
class Member(models.Model):
user = models.OneToOneField(User, on_delete=models.CASCADE)
firstname = models.CharField(max_length=32,default="",verbose_name="First name")