Fire changes in a user field -- use a surrogate field?

I am struggling infinitely with what I feel like aught to be so simple. It may involve pipelines, which I haven't broken into yet.

I want to log changes when a field changes (assigned staff member). But first off - you can't save changes in a User field. Second -- I've noticed when you log edits, the field then displays on forms as the entire string of changes..... so I've often thought, if logging is important AND displaying on a report (or filtering to charts) cleanly is needed, then it's probably best to make a surrogate field.

So here I am trying to make two fields -- 1) Current Staff Assignment, and 2) Staff Assignment History.  The former triggers a change/logged edit in the latter. But I have no idea how to go about doing this. I was going to do it with Form Rules, but for multiple reasons (chiefly, that I'm using multiple forms depending on staff/use), I want this to trigger at a more global level. How can I achieve this?