First everything in any practical simulation is always and everywhere an approximation. An exact method is an enormously stupid idea - a huge waste of resources.

We haven't seen anything like evidence that our laws of physics are only approximations at all. If we're in a simulation, this implies that with high probability either a) the laws of physics in the parent universe are not our own laws of physics (in which case the entire idea of ancestor simulations fails) or b) they are engaging in an extremely detailed simulation.

The optimal techniques only simulate down to the quantum level when a simulated scientist/observer actually does a quantum experiment. In an optimal simulated world, stuff literally only exists to the extent observers are observing or thinking about it.

And our simulating entities would be able to tell that someone was doing a deliberate experiment how?

The limits of optimal approximation appear to be linear in observer complexity - using output sensitive algorithms.

I'm not sure what you mean by this. Can you expand?

The upshot of these results is that one cannot make a detailed simulation of an object without using at least much resources as the object itself.

Ultra-detailed accurate simulations are only high value for quantum level phenomena. Once you have a good model of the quantum scale, you can percolate those results up to improve your nano-scale models, and then your micro-scale models, and then your milli-meter scale models, and so on.

Only up to a point. It is going to be for example very difficult to percolate up simulations from micro to milimeter for many issues, and the less detail in a simulation, the more likely that someone notices a statistical artifact in weakly simulated data.

We already can simulate entire planets using the tiny resources of today's machines. I myself have created several SOTA real-time planetary renderers back in the day.

Again, the statistical artifact problem comes up, especially when there are extremely subtle issues going on, such as the different (potential) behavior of neutrinos.

Your basic point that I may be overestimating the difficulty of simulations may be valid; since simulations don't explain the Great Filter for other reasons I discussed, this causes an update in the direction of us being in a simulation but doesn't really help explain the Great Filter much at all.