This is excellent commentary, but I would argue about your interpretation of Mirror Garak. In her series of blog posts about DS9 @abigailnussbaum argued that one of Garak’s central tragedies was that he was someone who believed in the ideals of his society, but was smart enough and cynical enough that he could never believe in them wholeheartedly. I would say that Mirror Garak is someone who either never had those doubts or taught himself to ignore them, becoming the good little soldier who steadily advances in rank even if his society marches him off a cliff. It’s not for nothing that Mirror Garak is a boilerplate Cardassian military goon instead of an Obsidian Order agent. (Additionally, Andrew Robinson noted in the DS9 companion that he played Mirror Garak as a “toady opportunist,” and he never had that much affection for that iteration of the character.)
One thing I treasure about this parallel in TOS is that it reminds us that being a “good person,” while it can ultimately become part of our being, is a series of constant, difficult choices. We are shaped by our experience and circumstances, and choosing to do good is more difficult in some situations and for some people than others.
It’s much more admirable knowing that Jim makes the choice over and over again to do the “good” thing, despite everything that has happened to him, so that he becomes the person the people who know him best think of as having an unassailable goodness, rather than just making a simple claim that some people are born to goodness and some are not. This extends to every character on the show, and into our own lives.
Goodness is not who we are, it’s what we do. Eventually, those actions become “what kind of people” we are. Knowing the other choice was possible makes the decision more meaningful.