Let's say you have the option of leaving secretly, so that no overt
confrontation is necessary whether you take your stuff or not. ...
I think this sends the message that what is wrong is metaphysical and therefore on a higher moral plane.
With all due respect, I strongly disagree. I think this sends a message of disrespect and cowardice. If you have loved someone, lived with them and shared their life and you can't bring yourself to give an honest account of your leaving, well, then I think the other person will probably feel abandoned and deeply betrayed.
If you really don't care about things, and you want to do the morally correct thing, you can simply explain this: "I don't begrudge you these artifacts of our life together and I'll happily leave them with you." You can certainly head off any "overt confrontation" by saying that the morally honest thing to do was not to leave like a thief in the night, and you hope the partner will respect this by not making the leaving unnecessarily difficult.
Of course, there are exceptions: if the person you are living with is violent, or if you otherwise fear for your safety, for example. But in that case, "things" aren't really an issue at all.
