I didn't know it existed.
Per device key derivation is interesting, but I don't know how much it is needed in this case.
I want that personal notes are saved on a relay that allows only the author to req them, so the privacy improvement protects just from relay owner's eyes.

In addition: how many derived keys should I take in consideration when subscribing for events? Maybe it's not a problem to use a 50 items list, but it's arbitrary, and this aspect could compromise the UX.