the image field stores and image id, a text field stores text. Even so, your theme is still going to output the featured image even if you find a way to add your other image field and you’ll still need to know how it is added in order to remove it.
To use an image field in a plugin where you can specify a meta_key for that other plugin to use you must assume that the plugin will not call an ACF function to get the value and will just call get_post_meta(), meaning that you’ll only see what is stored in the database. For an image field you have to assume that this is what is returned when setting the return value for the image field to “Image ID” https://www.advancedcustomfields.com/resources/image/
