uncut berry
You don't mention which of their hundreds of packages you're talking about, but in this case I don't think it matters. Filesystem permissions for Storage disks are controlled by your own config/filesystems.php
But it can be a bit confusing, because the operating system and php config can further limit the permissions that Laravel asks for. So even if you ask for a new directory that is group writable, the end result may be one that is not. This is called "umask".
I think generally the best and also easiest solution is to make the web server run as the php-fpm user. Then you won't need to have any group permissions.
wary spear