How can I free up space to reinstall macOS (Catalina)?
Ok, so I found this: https://support.apple.com/en-us/HT201573

Single-user mode


Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery.
Select Disk Utility from the Utilities window, then click Continue.
From the Disk Utility sidebar, select the volume that you're using, then choose File > Mount from the menu bar. (If the volume is already mounted, this option is dimmed.) Then enter your administrator password when prompted.
Quit Disk Utility.
Choose Terminal from the Utilities menu in the menu bar.
You can now enter UNIX commands. When done, choose Apple () menu > Restart.


The key is step number 4 &amp; 5, which allowed me to launch a terminal window while the correct volumes were mounted. I am currently re-installing macOS.