I want to make a form that once submitted, on each next rendering of the form for the same user, the form will be already filled out with the data from the last time the user su
This already sounds like a good plan. But if there is no pressure on using cookies, I would resort to using the session instead. That way you won't hit a size limit on cookies, and transmitting cookies with higher size can slow down your response time.
form = MyForm(initial=request.session.get('form_data'))
...
if form.is_valid():
request.session['form_data'] = form.cleaned_data