This, in all likelihood has been covered, but there are such a plethora of options that I’m feeling overwhelmed by choice. Options are good, except when there are too many of them. 
Also, I’m old, and inclined to favor the familiar, so rsync in a crontab entry would be my comfort zone favorite, but probably not the best in terms of someone else figuring out what I’ve done via DSM.
I’m currently in a copy and paste project to simplify the directory structure and copy the contents of the DS-1517+ to the DS-1522+. They are currently located within inches of each other. Once the files are copied over to the new NAS, and the shared folder structures on both match each other, I want to change to primarily using the DS-1522+, with all file additions, deletions, and edits happening on the DS-1522+. I want to routinely mirror the contents of the DS-1522+ to the DS-1517+.
I am also a huge proponent of K.I.S.S., which may also come along with being old. My two NAS are personal media file storage devices. The shared directories will be; audio, video, photo, and reading (since my epub library wouldn’t fit under audio files, video files, or photography.) The files will not be subject to much change, so my feeling is I could power up the DS-1517+ once a week or so and mirror any changes from the DS-1522+ over to my backup NAS.
I’m looking for a balance between simple and safe. I’m not a business, and I don’t need a ton of options. I just need to do a reliable mirror with the files on the backup device readable if someone were to log into the backup device.
I think part of my hesitation is that ‘back in the old days’, backup software wrote off the backup into a proprietary structure, and one needed the backup software to recover anything inside the backup. I do not want something like that. I just want to copy a file from the primary to the backup if it doesn’t exist on the backup, and delete the file from the backup if it no longer exists on the primary. Simple stuff. And if the primary crashes and burns, then I’d be able to log into the backup and turn it into my primary media server without jumping through a bunch of hoops.
I hope I haven’t been too wordy, but I wanted to give enough background to what I’m looking for that any suggestions would cover what I’m looking for. I don’t want to say, ‘I’m looking for this green thing’ and have someone come back and tell me how well a blue thing works for them. It’s great the blue thing works for someone else’s needs, but I’m still looking for the green thing.