So the default library installation on macOS is into /Library/Application Support (not ~/Library) which I think you picked up on. The space in ‘Application Support’ also seems to cause some problems. This location is to try and protect unwary users - as it is a write protected directory so you don’t store your own assets in a directory that might get wiped on the next update but it does cause some problems and may not be ideal.
How you play this depends a bit on how you use KiCad. I have cloned the KiCad libraries to a more accessible location and can update them easily with a git pull. I only use two machines so it is easy to keep them both up to date and in sync with occasional git pulls. Since the stock libraries are quite large, I choose not to keep them on Dropbox. If more people are using Kicad in your setup, a local file server might be a better location. But wherever you keep these assets, you can adjust the path to them in the ‘Configure Paths’ dialogue.
This is a serving suggestion that works for me.
I have a separate directory for my own assets which I keep on Dropbox. This is much smaller and contains my own symbols, footprints and 3d models. This is accessible from both machines (but would also scale to more if necessary and I have write access to this directory.
Whenever you adjust a stock asset, it becomes a personal asset so needs to be saved into your personal library which needs to be in a location that you have write access to. Check how you have your paths set up in ‘Configure paths’ and check that you have write access to wherever you chose to save your assets.
There is some helpful info in the FAQs esp Library management in KiCad version 5