Take your Library L and duplicate it. You now have L1 and L2.
So immediately you require twice the disk space.
From L1 delete everything before, say, 2015; from L2 delete everything from 2015 onward. L1 and L2 are now half the size they were
So you're working on the only complete copy of the library? There's no contingency there if something goes wrong?
The theory is correct, but in executing it there's at least one point where you need a minimum of twice the disk space. I was assuming you'd keep one copy as a back up and work on duplicates.
Edit: a thought: using PowerPhotos it would be possible to create a new library on the external and move material to it, with no duplication. Though, again, it's not wise to do this without a back up.
https://www.fatcatsoftware.com/powerphotos/