Where can I find all form display options types for custom entity fields?
Those are field widget plugin IDs.

You can get all of those by querying the field widget plugin manager (service ID: plugin.manager.field.widget).

E.g. to get all the IDs in your entity class (once the service is injected):

$widget_types = $this-&gt;fieldWidgetPluginManager-&gt;getDefinitions();
$plugin_ids = array_keys($widget_types);


Same thing in procedural code in case someone wants it:

$widget_types = \Drupal::service('plugin.manager.field.widget')-&gt;getDefinitions();
$plugin_ids = array_keys($widget_types);


If your current context allows you to inject the widget plugin manager service rather than get it from \Drupal, all the better.