Hello. I want to suggest a new form field type that is called form_type_modal.
Actually its a relational modal that can be created run time.
So when user select a relational user from drop down that time he can also create a new user in using create button inline.
So we can manage
n-n
1-m
m-1
m-m
Relational Modal type
For more details
https://sonata-project.org/bundles/admin/3-x/doc/reference/form_types.html