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.
