Not sure if you've solved this problem but...
I do a lot of tech work on macs and I wanted to give you some helpful suggestions in case you hadn't solved this particular issue.
First, the files through which you are navigating are only the ones Apple has not hidden from you. There are a myriad of hidden files which may be holding this mysterious amount of unused SSD space. Open terminal and type the following command exactly as written (including the capitals) hit return:
defaults write com.apple.finder AppleShowAllFiles TRUE
Then type (and return):
killall Finder
You should now see shaded files throughout your finder, you'll know you've done the steps correctly if you see .DS_Store and .Localized files on your desktop (this is because all folders contain these items and your desktop is essentially a folder).
Hold Command+Option and i to view the inspector and scroll through all your folders and check the file sizes (the cmd+optn+i makes it so you do not have to hit command+i on every single folder).
MY GUESS, is that you likely have a giant sleep image file sitting in your ~/Library/Var folder. My understanding of this file (and users, correct me if I'm wrong) is your machine creates a giant file of everything your mac is doing so that when it goes to sleep it can wake up, and have everything immediately ready to go from the moment it fell asleep. Sometimes the file can get corrupt and begins to build and build upon itself. In this scenario you have to delete the darn thing and let your Mac create a new one (which it should be doing every single time it wakes/sleeps). I once had a sleep file on a customer's machine at around 60GBs who had the same problem as you.
If the issue persists (either continues to create giant sleep files, or indeed you cannot seem to locate all your data and I am wrong) I would agree with one of the previous posts: Wipe and re-install. Fresh software is ALWAYS going to be the best and safest path to take. Because there is likely a problem in the system that cannot be fixed by these kinds of palliative solutions. Likely in the kernel or some some launchd process, which takes the issue to an engineering level haha and thus waaaaay over my head and not worth the effort.
So back up, wipe the system clean, then drag and drop your data (music/pics/movies/apps). I'd recommend NOT migrating your data over the easy way (using migration assistant with your Time Machine backup) since the issue may just be carried over with your old data.
Best of luck. Sorry for the late response, hopefully you were able to resolve the issue.
OH, one more thing, to hide all of the files you've just unhidden, use the same terminal commands I posted earlier but instead of typing TRUE use the opposite, FALSE. Boom, files hidden away from the user
Cheers!