Maybe have a look at Duplicati (
https://www.duplicati.com/); its a free backup agent that you can run on macOS (and Win/Linux) - it may even be available to be installed directly on your DS418.
It works with Backblaze B2 and bunch of others and it may give you that intermediate control to let you have more control of how the backup functions.
I've not used it myself though I see it pop up often in online forums. I'm still with Google Drive though I haven't accrued 7+ years of images!