The only reason that there is only 12.74 GB left on the SSD is because the High Sierra install crashed in the middle of installation, so all of the temporary (but incomplete) files are there, which no longer allows enough room for the install. I use an external drive for just about everything else, and I backup/TM to another external drive. My solution is to boot the macbook in target mode from my other macbook, copy and delete the install files, reboot in recovery mode, and do a new installation. I'll let you know how it goes, but I still wonder what made the initial install crash in the first place (there was plenty of room at the time).