I think there are two things mixed together here. A system to track the outcome of investigations (no matter the source) in a consistent and centralized manner (simmilar to the notes system on the server). And a system to confidentially report things.
Peronally I am not a huge fan of the current system which (in some cases) encourages players to DM a staff member with their grievances (be it a headmin/dev about staff behavior or a admin due to player behavior).
The problem with it is, that it is often not clear who has investigated what, what evidence has been investigated and what the resolution was. This is further complicated if the handling person retires from the staff team.
Even if the investigation, and its outcome is communicated to other relevant people (I.e. Me pinging skull about something I looked into) then that information will most likely be forgotten after some time has passed due to the nature of discord.
So I do agree that there should be some sort of system to (at least) track the investigations that occurred, their process and outcome in a orderly manner. (We have something like that already as the staff complaint archive, but that is not the easiest to navigate and only works for staff complaints)
The biggest question is if confidential reports are worth implementing. (As quite often a person will be able to detuct from the context who complained about them. Which is the main augment for confidential complaints.) Personally I could see such a system being implemented, however with a much stricter "access control". (I.e. In addition to the usual staff complaint format, having to specify why it is not possible to report something as staff complaint; and if there is insufficient reasoning, such a report is rejected)