Have get_the_post_thumbnail output different image if custom meta exists

I have a meta box on each post where a user is able to select another image from the media library.

I want this image to override the output whenever get_the_post_thumbnail is used across the whole theme.

I’ve looked through the wp-includes/post-thumbnail-template.php file and found the get_the_post_thumbnail function, and it looks like this might be what I want to hook into but I’m not exactly sure how to use it:

I want to do something like this:

I have a feeling it might not be possible with an action, so I noticed there is also the post_thumbnail_html filter, but I was hoping to avoid having to modify this if all I want to change is the attachment ID and otherwise keep the HTML exactly the same.