Mac won't update even after restart? Use the terminal!

I saw that there were some new updates to MacOS Catalina, so I tried to install them through System Preferences / Software update:

After trying to get the update to install (forcing a restart), I noticed that it would not install — even after multiple tried with full battery and plugged into power.

After a while I assumed I didn't have enough free space, and tried again — still no luck.

I found somewhere a command to install the updates from the terminal, so I tried it:

sudo softareupdate -i -a

Then the truth was revealed:

There is nowhere in the Software Update prefpane that states how much space is required or what is blocking the update. This should be a relatively simple UX update that will allow less technical users to be safer.