Display entity as options_select multi-select
The options_select widget will automatically be multi-select if the cardinality of the field is multiple. Make sure your field is created as such, e.g.

use Drupal\Core\Field\FieldStorageDefinitionInterface;

...

$fields['foo'] = BaseFieldDefinition::create('entity_reference')
  ...
  -&gt;setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
  -&gt;setDisplayOptions('form', [
    'type' =&gt; 'options_select',
    'weight' =&gt; 0,
    'settings' =&gt; [],
  ])
  ...


Incidentally, options_select has no settings, so you can provide an empty array instead of the autocomplete settings you're currently using.