Help backing up android tablet (nexus device/no microsd).

I have a 32gb nexus 7 2013 with only 100mb left of free space. I have titanium backup, and have done app +data backups to a removable thumb drive with OTG cable. How can I back up ALL my directory data etc. so I won't be missing any documents and media when I wipe the device? What I need is something like a Macrium reflect image of the device, or some other way to reliably grab everything so I can restore what I need after a re-install. I need to install a previous version of android (4.4.4). I posted more specifics and background on why I need to kill lollipop with fire in the nexus 2013 forum
Thanks!