This feature doesn't work for widget : inline entity form - Complex

Yes you are correct there is no field widget defined for IEF. You can try to create the file in the src folder and extend the existing widget. It's probably a lot like the others.

With that said I will create a branch for the work. But one thing I wanted to do was configure tugboat so it's easier to develop for all. I will try and do that tomorrow so we can get a test environment setup.

I have created a patch for the same.

This… does it, but Inline Entity Form Complex is… complex. It does not seem to be a good idea to take into this module the burden of keeping up with its 1,400 line formElement method.

Is there precedent for third-party modules choosing to take Field Config Cardinality into account if the module is enabled and a bundle-specific cardinality is configured? Issues where this was explained to maintainers? Examples of what that code looks like?

I've done this integration for IEF Complex Open Widget → here: ✨ Integrate with Field Config Cardinality Active

Replacing this line:

    $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();

with:

    $config_cardinality = $this->fieldDefinition->getThirdPartySetting('field_config_cardinality', 'cardinality_config');
    $cardinality = isset($config_cardinality) ? (integer) $config_cardinality : $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();