Multiple Instances of Entity Form
Here is what I would do:

Create that many entites as you would like to able to submit with entityTypeManager (members)

Create their form objects with EntityFormBuilder-&gt;getForm($entity)

Set out all the elements you don't need (their submit buttons)

Add them to form like this:
$form[$custom_form_id] = array(
'#type' =&gt; 'markup',
'#markup' =&gt; render($created_form),
'#prefix' =&gt; '',
'#suffix' =&gt; ''
);

In your submit, call the children form submit functions.