From some quick googling, I agree with you. Finding a free utility that does things "right" (especially for SSDs) seems non-trivial right now.
Surprisingly, it IS trivially easy to securely wipe your sensitive data, even from SSDs, assuming the system can successfully boot OR the drive can be successfully attached to a system that can boot. It just takes time and requires attention.
To permanently delete your sensitive files using nothing more than the OS itself, do the following.
INTERNAL
-Log into each user account in turn and uninstall all programs and sign out of all online services/websites (bank sites, Amazon, iTunes/Spotify, Facebook, iCloud/Dropbox/GoogleDrive, etc).
-Clear all browser content, empty all caches, temp files, downloads, documents, pictures, etc.
-Empty the Trash/Recycle Bin/Black Hole, or whatever it is your OS of choice decides to call that place where you put the stuff you’re throwing away.
-Create a new administrative user account and log out of your account and into that one instead of any of the users whose data you are going to remove.
-Use whatever account tools let you delete the other users off the computer. If you are asked whether you want to save any of that user’s data/settings/whatever, say no.
-If the OS hasn’t already done so, trash the deleted users’ folders and empty the trash/recycle bin.
-Alternatively, erase the drive and restore a fresh empty copy of the OS.
EXTERNAL
-Lasso everything on the external drive that you are clearing and trash it, and then empty the trash, or just format the external drive.
BOTH
-Once you have deleted all of the things you want sanitized, by whichever method above:
-Download a big honkin’ file of some kind. Maybe it’s a giant game demo or the Warcraft free trial, or record a 20min audio file, or whatever. Just make sure it’s huge and that it doesn’t contain any sensitive data.
-Duplicate that giant file/folder over and over again until it fills the drive.
-Delete these duplicated files.
-Fill the drive with garbage again and empty it repeatedly until you feel like you have sufficiently obliterated your sensitive data.
-Your drive is now clean, it was just more labor-intensive.
—Patrick