As I originally outlined in this comment there is a work around for the problem of having shared credentials and keys on the various backup "clients" so that they push their contents to your restic server.
I have implemented this solution, but it took a fair amount of work. I am putting this work up for bounty, so if you are interested please contact me to arrange payment and when the goal is reached, I will send my solution to everyone who chipped in.
Thank you very much!