You are not going to be able to add an image path to an ACF image field. ACF image field like you say, only store the image ID. All of the other information is stored with the image’s attachment post entry an post meta.
To do this you need to insert the image into the media library and then update the ACF field with the ID of the attachment. This is not something that ACF will do for you.
See https://codex.wordpress.org/Function_Reference/wp_insert_attachment.
The other information is stored in various places
alt => post meta name _wp_attachment_image_alt
caption => post excerpt
descriptions => post content
title -> post title
