Increasing a user's home directory size, external drive?
Moving the entire user domain, instead of just the folders for Videos and Photos may be overkill, though it would keep things simple. You're not going to see any advantage from moving preferences, audio files, Word docs, etc, as they are too small and not causing any bottleneck.

There's no disadvantage either, except that if the Mac boots up and can't find the drive, it will create a brand new (empty) user account on the system disk. "Where has all my stuff gone?" 

Don't forget that you'll need another device to back up to!

Fusion drives can reduce in performance as they fill. I would recommend swapping one or more parts out for a regular SSD in due course.