Per usual when the OS gives up you have to boot to the recovery partition, mount the problematic disk and work it from outside.
A hard shutdown was required, and held down the power button for 5+ seconds.
Right after pressing the power button, held down Cmd+R until the apple local and progress bar popped up.
After Recovery mode came into view, in Disk Utilities, mounted the "Mac Harddrive" partition.
Opened a terminal window, and cd into /Volumes/Mac Harddrive and then started stripping
large files out of the downloads folder.
Removing old Parallels VMs helped. Cleaning up large movie files helped.
du -sh ./* # show folder's disk usage
du -sh ./* | sort -h
ls -lSr | tail -n 10
brew install baobab
baobab &
df -h
docker system prune
WinDirStat
CCleaner