To enable writable, you have to set up samaba in /etc/config/samba and /etc/samba/smb.conf.templete combined. I think you have done it right in step 4. but I didn’t see you set /etc/config/samba. Maybe luci is not very reliable in settings.

But if you are using v2.22, why not use our UI? There is settings in our UI - Sharing and you can just enable writable.