Ok. So like many I have faced the iPhotos vs Photos conundrum ... the issue of managing an evergrowing Photo.Library eating my HD space, the issue of migrating all my iPhoto libraries on external drives to Photos and the ongoing issue of creating and managing multiple Photo.Libraries on external HDs when only one can be synched via iTunes....
I have browsed many forums and discussions and even spoken face to face with the local Geniuses and sadly found that although the Geniuses can be very helpful in debugging and installation issues they just did not have the experience or best practice tips on how to run iPhotos/Photos over an extended period of time.
However thanks to this discussion - and in particular the clarity of Terence Devlin's and Old Toad's posts .... and the discovery of another utility PowerPhotos (from fatcat software) I have decided to embrace Photos as my path forward. Terence and Old Toad's insight allowed me to bridge my workflow from iPhoto to Photos and hence stay aligned with Apple's direction, PowerPhotos gave me the ability to manage multiple libraries on separate devices - split/merge libraries, even migrate the iPhoto libraries I had on external devices to Photos format etc etc. It was the missing link for me that made Photos workable without having to commit to a full blown DAM such as Lightroom etc.
I can now have a "main" or default Photo Library on my HD which syncs with my other devices via iTunes or iCloud and it is the default into which I can upload all my new photos and videos. I can then with PowerPhoto drag/drop Albums, Photos, Videos etc out of this default library into other Photo Libraries on my HD or external HDs so that I can manage my iCloud or HD disk space. I can have all my libraries visible and accessible at the same time so that I can see what is inside them and decide where its best to move the photos. This works for me.
I understand that a DAM offers much much more, but reading the many discussions I see that many people are like me, we just want an easier way to manage our family snaps and videos and at the same time stay aligned to Apple's direction. I am posting this here in the hope that my summary helps someone else avoid spending the days I have spent on researching this topic.