If I am commanded regarding something, but I know that there will be no enforcement of it in any way, and no reward for it, why should I do it? [If not in order to be a good person—but not all the commandments are understandable by reason alone].

Also in civil law, for example, if Sunday were a mandated day of rest—I would not observe it without enforcement.

Of course, I am not speaking after the commandments of love of God and awe of His exaltedness, not for the sake of receiving a reward—in which case the question is invalid from the outset. (And perhaps that is what you meant.)

You are not asking whether it is binding, but why you should obey. That is really not the same question. Obedience means doing what is true because it is true (in order to do what is true, not in order to be a good person). Reward and punishment do not make it binding; at most they give you motivation to obey.
See Maimonides, beginning of chapter 10 of the Laws of Repentance.

A proper halakhic response.