I would delete all your own data folders (having kept them on another drive). Do double check for passwords etc. Once you have deleted all your stuff, I would use CCleaner drive wiper using "wipe free space only". You can choose the 3 passes or 7. Both take a long time - ? overnight? Unless you are very paranoid I would have thought 3 is enough. You could leave a copy of "Recuva" on the OS then , after the wipe, see if you can use it to recover a sample file.
HTC U12 Plus review: Hands-on