I have an “set as default” option to a custom post type but I only want that option to be available or not disabled if no post has been marked “as default” previously.
So a sort of conditional logic but across all posts in my post type.
Is that possible and if so, how would I go about it?
I would suggest that rather than having this setting on every post that you instead create an options page that has a post object field to allow for the selection of the default” post.
To do this on every post you will need to create an acf/load_field filter https://www.advancedcustomfields.com/resources/acfload_field/ and do a query in this filter to see if there is another post that already has the value set and if so then disable the field.
