I had to make a plugin instead.
In order to obtain the post_count value when viewing a post or a user’s card, it is necessary to serialize the data to include this field. This can only be achieved with the help of a plugin and relying on requests per user is not a realistic solution.
Give it a try. The JS part is in assets if you want to take a look.
One specific thing you may have missed in using the API is the ability to customize via outlets.
Let me know if you want more options.
I don’t know if I should release this plugin as it’s small, but here we go anyway: