Thanks for some insights, @pazos! I actually used a find similar to yours, twice. First to be sure I found the right ones, second to actually erase the sdr folders.
Unfortunate, though: Since I was at home, it also looked at many TB of NFS automounts across the WiFi, making each run take roughly half an hour. ;-) And of course I had also tested some of the EPUBs on those mounts.
Anyway, now we know more!