I don’t think you’ll be able to achieve that with systemd paths, I’m afraid. It’s not a use-case it is designed for.
It’s hard to come up with a suggestion without knowing more about the depth of the directory and the number of nodes in each level. But you could try updating a dummy file such as latest_timestamp in the top-level directory (which a systemd path can monitor BTW) and let the service unit be triggered by that.