Mojave is eating storage like candy, no matter how much I free up
You might want to check the disk usage with something like 

du -h 

which will scan the current working directory.

or 

du -hs if you have an idea which directory might be taking much of the space.

OmniDiskSweeper is also an awesome free tool to see where all the files are taking so much space but be extra careful with it as you would not want to remove any system files, here is a video where you may get some ideas.

Another reason can be swap memory. When there is a lot of applications running and the system memory cannot take it anymore, it saves temporary memory into ssd, as a result when you are rebooting, it is going away but then the system is making it again. You can check your memory usage by:

vm_stat | perl -ne '/page size of (\d+)/ and $size=$1; /Pages\s+([^:]+)[^\d]+(\d+)/ and printf("%-16s % 16.2f Mi\n", "$1:", $2 * $size / 1048576);'


Or by:

sysctl vm.swapusage
&gt; vm.swapusage: total = 5120.00M  used = 4237.00M  free = 883.00M  (encrypted)


For more info: Is there a Mac OS X Terminal version of the &quot;free&quot; command in Linux systems?