We had a bit of a discussion on this recently and I highly recommend against trying to keep your media on a separate drive, especially
with a laptop. I also agree that going through and trying to pare down that media would be time-consuming and very annoying to do.
But before you plonk down on disk you may or may not need, I’d suggest running the free app GrandPerspective to analyze your data. It may turn up nothing to delete that’s huge enough to make a difference, but in my experience, it always finds a lot of data I don’t need to keep that I didn’t realize I had saved.
I just ran it on my documents folder and found a 36GB video file (an interview with Nobel prize winner Dr. Andrea Ghez). I want to keep this file forever, but there’s no reason it has to stay resident on my internal drive.
There are a lot of apps that provide this functionality that you can use, but I’ve been using GrandPerspective since 2007 and it’s in the Mac App Store now and does a great job.
The only trouble with these apps is they see Photos as one giant blob, but it might give you “perspective” (get it?) on how big it is compared to other things you might have stored locally.
It sounds like you must not be using iCloud Photo Library if you’re only paying for 50GB of storage.
Curious to see what you find.
Personally, if I could afford it I’d buy a 2TB MacBook Air (in fact that’s exactly what I did) rather than trying to keep media on an external drive with a laptop.