Yes, as far as I know there is no way to “unattach” the image from a post (you can attach new ones though, via the “Uploaded to” column in the “Media” admin menu). You can try using this third-party plugin for detaching images from posts.
And yes, it would be so much easier to source the images from an inserted gallery, yet during theme development I haven’t found a way to tell the php code to look up the images from an inserted gallery.
Sincerely,
Andrew / Satori Studio