If a sentient entity could calculate the future via determinism, it is forced to either follow it or break it. Which would therefore lead to free will and break the whole concept.
I have to disagree with you here, philosophical determinism in the Philosophy of Mind is radically different than physical determinism.
Let me explain, you are committing an equivocation in what I marked in bold. Since you argue using the word Determinism (in a physical context), and free will (which is the opposite of determinism in a different and philosophical context).
If a sentient entity could calculate the future via determinism (this is a physical determinism), it does not follow that free will will such and such...
Because your argument groups two concepts that are not related, they are not even opposites, since they belong to two different domains of philosophical investigation.
Physical determinism today can be refuted by evidence from Quantum Mechanics, you can never calculate the future via physical determinism even if you know everything about all the atoms in the Universe.
But this does not say anything about Philosophical Determinism or free will.
Physical Determinism : Everything in the Universe can be predicted, given that you know a current state of affairs, you can predict the future.
Physical Nondeterminism: Given that you know every singe state of affairs, you cannot predict the future of the Universe, since everything in the quantum world relies on probabilities and not on deterministic laws.
Philosophical Determinism We do not have free will, our decisions are determined by physical factors and we have no control over them.
Quantum Theory refutes the first idea (Physical Determinism), that is, you cannot predict what will happen in the future even if you know everything.
But this does not mean that it refutes the philosophical determinism, since, it is possible for nondeterministic quantum phenomena, to give rise to physical states of affairs, causes and effects that contribute to our decisions, which means that we still do not have free will, since our decisions are based on what happens in the non-deterministic quantum world.
So why are we thinking about this topic so much?
Is there anything we could gain from it?
Yes, there is something we gain from it, and it is better understanding and knowledge (or at least, understanding what the questions are all about), that is, in my opinion, the most sublime of all gains.
I suggest to read : https://plato.stanford.edu/entries/freewill/#FreeDoOtheVsSourAcco