The GUID is not the image’s URL. It typically does end up (coincidentally) being the original URL of the image, because it needs to be a unique identifier – but that’s all WordPress uses it for, as a unique identifier.
Your code should successfully update the URLs of the featured images of posts, but only the featured images. It sounds like you are probably running into references to the images inside the post_content itself (as in, in the (prefix)_posts table, in the post_content column of each individual post and revision). This happens both with the Classic Editor and the Block Editor – the full image paths are included in the <img> tag.
So, you’ll have to update all of the Posts/Pages/CPTs that include images. This will probably be easiest in PHP, where you can get all posts, loop through them with a regular expression, and replace the image paths programmatically. (If you’re running a smaller site with just a little old content, it might be faster to do it by hand.)
But, there’s one more place in the database that references the location of the image – the URL that you see in the Media Library, so WordPress knows when you add the image to a new post it will add the right path. That’s in the (prefix)_postmeta table. For each individual image file, you’ll find one entry in (prefix)_posts where you can grab the IDs. Then, for each of those IDs, there will be two keys in the (prefix)_postmeta table that refer to the path. The “_wp_attached_file” postmeta stores the path and file name as a simple string. More complicated to deal with is the other one, “_wp_attachment_metadata”. This stores the path and file name inside a serialized array – meaning you can’t just search and replace strings, or it will break the serialization. So, it will take more doing to find a way to update that postmeta value in a way that causes WordPress to look for the image in the right place.
Really, if your image library is small, you may want to even consider uploading all of the media freshly through the Media Library into their new location, and then going back and manually updating posts (though of course if your site is larger, this will be rather time-consuming, so you might want to figure out the serialization instead and do things programmatically.)