Standard Wordpress thumbnail input may be replaced with an ACF image field which is far more flexible/customizable.
In order to do that you just have to give it a name of
_thumbnail_id. I'm not sure if it is possible with a visual editor (because I don't use it), but for sure you can do that using ACF PHP API. Here is the code which will generate a field for thumbnail image selection:
array( 'key' => 'field_thumbnail_id', 'name' => '_thumbnail_id', 'type' => 'image', 'label' => 'Custom thumbnail input', 'instructions' => 'Some more instructions below the label', 'return_format' => 'id', ),
Of course all other image field settings might be used here e.g.
max_size etc. For more info please refer to the official image field documentation at https://www.advancedcustomfields.com/resources/register-fields-via-php/.