I think the Solar System could be a simulation, but the universe could not be. I also don't see a reason to think our Solar System is either. The universe though? No way. There several reasons why not.
To say time exists outside of the universe is to say that the universe is not really the whole universe, so its just denying what universe means. Universe means everything, including space and time and matter and all of history and the future and every tiny bit of any spec that is a cousin of any other spec no matter how far away or long away or how long before. That what universe means. But we can suppose that time and space exist in something that isn't time. This is where philosophers usually go with it: Brahman, God, hypergraphs, information theories. Others simply rest with admitting they cannot definitely know or that there is not any data. The problem of simulating a universe is that your simulator is part of the universe, so you're saying that the universe isn't a universe. Philosophically that is disappointing.
That's why simulating a Solar System or a Galaxy is conceivable but not simulating a universe. Incomprehensible but not absolutely impossible.
It runs counter to the discovery that time and space seem to be related. Simulations require time, but time seems to be part of space. How then can the universe (which is space and time and other things) be a simulation? Its like saying that the minute hand is the watch that it is installed in. This is why most physics informed guesses about the nature of the universe tend to have a transcendant concept of the outside. We have hypergraphs, information theories, Brahman, God etc. These don't require time. Time is part of the universe. But if our Solar System or Galaxy are not real, then you could think of them as simulations. It just that philosophers aren't satisfied with that. We want to envision what everything is, to think about the nature of reality itself. A simulation gets in the way.