So that’s the test? Whether I would do it if there were no benefit at all? Seemingly that’s a hypothetical situation, because here the benefit actually exists.
I’m trying to understand why this isn’t considered fulfilling a commandment if my belief is that God’s commandment is to benefit the human being. He commands a person to do things that are good for him. So tefillin and Sabbath and so on follow from that.

This connects to Spinoza’s view that God is the totality of being—and being strives to persist—so “God’s command to man” is to strive to persist, to do things that lead to that. And since I belong to the Jewish people, which created an excellent way to fulfill this, I keep it.
Isn’t that called fulfilling a commandment?

The question is whether you do it out of commitment to the command or not. If you do it in order to improve the state of the world, that is a fine and moral act, but not a commandment.

I am proposing exactly to equate these two things: the command is the benefit, and the benefit is the command.
What I’m saying is that my personal motivation can be different on different days: sometimes it will be the supreme ideal of fulfilling God’s command to do good, sometimes it will be because of the benefit, and sometimes just habit. The motivation changes, but because of the identification of the benefit with the divine command, it doesn’t matter in principle.

I understand, and I see nothing here that is new relative to what I rejected. This is exactly what I came to reject.

So why? Because in your view the command is not for the sake of benefit? A command of “just because,” or because of hidden kabbalistic reasons?
What does one help over the other?

To say that He commanded because of “hidden reasons” is no more convincing than to say that He commanded because it is good for His creatures. Why not?

You are mixing different planes of discussion. I am not entering at all into the question of why He commanded. That is not the question we are dealing with here. For the sake of the discussion, I will accept that everything is intended to benefit us (although I very much doubt that; it does not sound plausible to me at all). Our discussion here deals with a completely different question: what my motivation should be when I come to fulfill the commandment (and not what the reason is that the Holy One, blessed be He, commanded it). Think about a child who does something because he will get a candy, while his father commands it for a different purpose.
Something like this appears in the introduction to Aglei Tal, which says that there is no problem at all with studying Torah and enjoying it (contrary to one who sees this as study not for its own sake), but it is problematic to study because of the enjoyment. Similarly, there is no problem in saying that the commandment is meant to benefit us and that I also do these things because of that. But it is problematic to do them only because of that (and the practical difference is: when I do not see a benefit, will I do it or not).