Validating a Django model instance with a model form without having a form data dict

demo.py

# We have many instances in our apps where the user goes through many steps of forms.
# Each step obviously has it's own ModelForm. At the end, there is usually a final step
# for the user to submit everything. For this step, we usually want to run all the other
# form validations again, just to be sure that all the data looks good.

# Problem: How do you run a form validation if you don't have any POST/GET data?

class Form1(forms.ModelForm):
  class Meta:
    model = Model1
    fields = ['field1', 'field2']
    
# Given this form, on my final submit view, I would want to do something like this:

def submit_view(request):
  instance_id = request.POST.get('instance_id')
  instance = Model1.objects.get(pk=instance_id)
  form = Form1(data=???????????)
  # See the problem? What is data? I can't pass the model instance as data
 
# POSSIBLE SOLUTION:
# So I thought, why not build up the data dict with three m ore lines of code:

def submit_view(request):
  instance_id = request.POST.get('instance_id')
  instance = Model1.objects.get(pk=instance_id)

  data = {}
  for field in Form1.Meta.fields:
    data[field] = getattr(instance, field)
  
  form = Form1(data=data)
  if not form.is_valid():
    # tadaaa :)
    
  # repeat the above for all other forms in the wizard

# Am I missing something here? Is this an antipattern?
# I can imagine some cases where this might fail, but I
# suspect that in most cases it would work?