Best cloud based photo storage
If you have not found a service that is allowing you to access all your photos on your iPad from all your different devices I would check out younity. younity allows you to stream or download your photos from any mac or windows device directly to your iPad. If you have external drives plugged into those devices it will allow you access to those photos as well. There are no storage limits as all your files are local. This product seems to have been made specifically for the situation you are in.
Hope that helps.