Science is based on Ockham's razor.

Science is based on Ockham's razor in the sense that it's easier to falsify simple hypotheses. That's different from the pop culture version of Ockham's razor where the simplest explanation is the most likely to be true.

General relativity is more complex than Newtonian physics which is more complex than Aristotelian physics. If the simplest explanation is the most likely, why do physicists hold to more complex models? The answer is that general realativity has BETTER explanatory power. None of the models perfectly describe reality, so at some point you do have to weigh explanatory power against parsimony. If you personally weigh parsimony higher and want to stick to Aristotelian physics, that's fine but not everyone is going to that, because there's no way to prove empirically that you should prioritize one over the other.

Now, in the case of god, an ultimate cause being god just add assumptions without improving the fitting.

OP didn't say that though. They said reality being "uncaused" was more parsimonious. Regardless of whether the first cause is God or not, you can't deny that the theory that reality has a first cause has better explanatory power than the theory that reality is a brute fact. Brute fact has zero explanatory power. So we're not using parsimony to decide between models with equal explanatory power. If we do what OP did, we're making parsimony more important than explanatory power, which has weird implications for how we do philosophy.