Determinism

[Purple Rabbit]

Determinism is just the idea that the future can only be one way, it can't go any other way, hence why I said it means there is "Only one possible future".

I don't agree. In science determinism generally refers to causal determinism:

Causal determinism is, roughly speaking, the idea that every event is necessitated by antecedent events and conditions together with the laws of nature.

You can observe from this definition that it does not claim "one possible future", only that an event of reality has antcedents that have necessitated it. IOW, this definition looks back into the causal history of events whereas your definition claims something about the future.

So according to determinism - if it was true then ultimately when it comes down to the details of the matter (whether we will ever know them or can know them or not) even the random quantum mechanics can't be any other way in that exact given moment.

Though this is not a result of the stance of causal determinism, since that strictly speaking allows multiple futures as for instance in a multiple parallel universes scenario, we experience just one future and from this fact of life (not from a scientific undergirding) from inductive reasoning follows that only one possible future (whatever it is) is relevant to us. It is fully determined in the sense that it will occur whatever it is.

At the moment quantum mechanics seems to defy much intuition, common sense and cause and effect and what not. But IF determinism is true then that would mean ultimately it's an illusion and if we somehow broke down the quantum into an even smaller quantum level then it IS ultimately all fixed and "determined" and only at the moment does it look "undetermined" or whatever.

As I've stated, the observation from experience that only one future is accessible to us (provided that we live long enough to be around) is not comprimised by the fact that we lack the precise physical description. The final theory to base this on is still lacking. Some descriptions of quantum mechanics, such as Bohmian quantum mechanics, indeed are fully deterministic. However when the so-called "quantum equilibrium” condition is met the Bohmian quantum description is equivalent to standard Copenhagen QM which is not deterministic. So the jury is still out whether we can come up with a fully deterministic description of reality. But this being so does not compromise the fact that one possible future is all that will result when we infer possible states from past experiences.