Increasing a user's home directory size, external drive?

Solution 1:

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.

Solution 2:

Do it!

I got a 2018 Mac Mini as a gift but only a 256GB drive. My user profile is almost that size. So I got a 1TB SSD and a nice USB-C/Thunderbolt case and copied the profile over there. Works a treat for me.

Once you have copied her profile to the external drive go into System Preferences > Users and Groups and Control Click on her user name and select Advanced Options. In there you can choose the user folder on the external drive.

Once that is done reboot and delete the old user folder.

With a nice fast drive with a quality case and a USB-C/Thunderbolt 3 interface she will not likely notice any slowdown at all.

Note that I didn't move the Applications (or any other) folder as I had enough room for those. While there is no officially supported way to move those folders to another location I believe you can still do it with a hard link. That is working for my Dropbox folder on another Mac just fine. And keeping everything else on the internal drive you should see little or no slowdowns due to system directories being elsewhere.