If you are on Windows, Bvckup 2 is pretty much exactly what you need.
Incremental propagation of modified files is not a big issue. Any backup/sync software can do it. The trick is to have the rename detection supported. There are two ways to do it - first is to parse the file system log (NTFS has one, for example) to see the actual changes, second is to scan both locations and then run some sort of comparative analysis to understand if any of the deleted files match any of the newly created ones. I don't know a single backup software that works with the journals, so the first option is not really an option. For the second option you will need to have a software that generates backup plans, i.e. it first scans the trees, then analyzes them, then spits out the list of steps to be performed (rather than reconcile the changes immediately at the scan phase, as robocopy /mir does it, for example). The issue here is the size of your backup. Very few backup apps won't choke on few million files, leave alone of 50 mil. I've used the above app successfully with a 6 mil backup and I suspect that it should be able to chomp through your case as well.