The current Samsung Magician software is now working on my system, so I'll be able to update the firmware in the near future. The one thing that is different now is a missing Adata SSD from the system, and installing the Magicina with admin privileges (although I think I tried this last time as well, I distincly remember making multiple attempts)
I'm still thinking on how to proceed though, as one final step to check if the drives are indeed having data retention issues is to refresh the data with the same firmware in place. If reads go up to a steady max value, there is no doubt we're facing the same issue (in terms of symptoms, not cause) as with the early 840 drives. On the other hand, if I upgrade the firmware and performance goes up, I'll need to wait another 3-6 months before I can make this test again (if ever).