How can I safely backup and remove Photos from MacBook (El Capitan)? Sub question: Can this be done without a great deal of manual nonsense?

I have over 100GB of Photos / Videos in my library after a couple years of imports from my iPhone. While I know that most of these have been backed up to Dropbox automagically, I really want to make sure that I have a true backup and then reclaim this space on my machine.

Apart from the fact that these things are apparently stored in more than one 'library' since the migration of iPhotos to Photos, and there's nothing that I have found that sounds a clear 'happy path' to accomplishing this goal short of manually exporting through what seems to be a rather laborious process. 

Perhaps I am seeing / making this more complicated than it needs to be, so if someone can recommend the best way to do this without having to spend hours and hours manually exporting things. 

Ideally I would like to simply connect an external drive and export everything in its original quality to said drive and then easily wipe them from my MacBook's SSD. Can this be done without fear of losing anything in the process?

My thanks to the community for any help that can be offered!
I have a simple solution for that:
Make sure to hold option key while clicking on Photos before lauching it.
Choose the folder where your different libraries are stored (if you want to create different libraries check this out
 Then, make sure to be in the Photos stack and select Command + A to select all the pictures. 
Head to File > Export (or click on Command + Option + E).
Don't forget to specify your external files format and videos definition.

Then you can choose to export your original or modified pics on your external hard drive.

Hope it helps ;)