We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Free File Sync can auto run when a sd.card is inserted. But you are probably going to want to give a unique folder name for each disk not sure how to do that.
I use a robocopy script to inject photos. Simply enter the drive letter and what you want the folder called.
I all likelihood having 6 SD cards reading at once is going to be slower overall than doing 3 batches of 2. USB kind of sucks that way.
Unless you are going to put 6 dedicated USB hosts in your PC.