It's not surprising that one particular parsimony principle can be used to overturn one particular form of theism. After all, most theists disagree with most theisms...and most believres in a Weird Science hypothesis (MUH, Matrix, etcv ) don't believe in the others.

The question is: where is the slam dunk against theism..the one that works against all forms of theism, that works only against theism , and not against similar scientific ideas like Matrix Lords, and works against the strongest arguments for theism, not just biblically literalist creationist protestant Christianity, and doesn't rest on cherry-picking particular parisimony principles?

There are multiple principles of parsimony, multiple Occam's razors.

Some focus on ontology, on the multiplication of entities, as in the original razor others on epistemology the multiplication of assumptions. The Kolmogorov complexity measure is more alligned to the latter.

Smaller universes are favoured by the ontological razor,but disfavoured by the Epistemological razor, because they are more arbitrary. Maximally large universes can have low epistemic complexity (because you have to add information specifying hwat has been left out to arrive at smaller universs), and low K. complexity (because short programmes can generate infinite bitstrings, eg an expansion of pi).

we basically already know how to write a program that implements physics-so-far-as-we-understand-it, but we seem to be some way from writing one that implements anything much like morality-as-we-understand-it.

Morality as we know it evolved from physics plus starting conditions. When you say that physics is soluble but morality isn't, I suppose you mean that the starting conditions are absent.