I don't know how Calvinists came to their conclusions. I can see how a scientist would come to a conclusion that the universe is deterministic by taking a too literal view of mathematics. That is why the indeterminism of quantum physics provided such a shock.
I can also see how someone who believes in "intelligent design" could be a determinist. They may be assuming the universe is a machine that God made. To the extent that intelligent designers believe in determinism, to that extent they are wrong. The very word "designer" is a poor metaphor for what God would be.
Why is it meaningless? If we made a choice yesterday and we have to face whatever karmic consequences for that choice today, we still made that choice yesterday. It was not determined yesterday. True we are constrained by those past choices but they were choices when they were made.
By making a choice we add something new to reality that cannot be explained by either determinism or a uniform random process (like flipping a coin). This adding something new is what bothers determinists.
Why "of course"? Are you assuming what you are trying to prove?
We have many influences over our lives. You have listed some influences:
(1) neurons: Do neurons make choices?
(2) God: Does God make choices that influence us beyond setting some initial state at the beginning of the universe? I think the assumption of this initial state is contradicted by quantum physics, by the way.
(3) our rational decision making: I assume we are not forced to accept the conclusions of that rational process.
One of the goals of determinism is to reduce a choice that we make to something that does not make a choice. It is the same thing with consciousness. Reductionism attempts to reduce our consciousness to something that is not conscious.
What I am saying is that we can make a choice for which we are responsible which is a sign that we are conscious enough to consider at least two possibilities and choose one of them.
The reason you are responsible for your choice is that other people insist you are responsible in spite of and in direct contradiction of the belief of determinists who claim your choice was an illusion.
On the one hand you have a determinist who says you cannot make any choice, you cannot add anything new in even a minor way to the universe that some mathematical formula or God's foreknowledge has not predicted. On the other you have people, including yourself through you own common sense, who insist you can and do actually make those choices. You are trying to put these two contradictory positions together as one which leads to a confused view of "choice". In reality we either can add something new with our choices or we cannot. Make a choice which one you support.
I am aware that people who label themselves "atheist" have differing views of reality. The same goes for people who label themselves "theists". There are times when I think the theist is little more than an atheist and vice-versa.
That doesn't mean I cannot challenge either of these positions.
What I am trying to show you are the walls of the cultural box we are both in. We both agree that we have constraints. Are all of those constraints something that we can use technology to overcome? I would say they are not. That is like a door I am currently using to point to an exit leading out of our cultural box.
A technological solution addresses what it controls as something unconscious. It itself is an unconscious mechanism (in spite of the views of artificial intelligence). So to rephrase the question, the door opening outside the cultural box: Are there forms of reality that constrain us that we cannot use unconscious technology to control?