You seem to be thinking in terms of time complexity. Space complexity also needs to be considered. It seems axiomatic to me that an outer universe simulation can only contain nested universe simulations of lower space complexity than than itself.

If I am wrong, is there some discussion of this kind of issue online or in a well-know paper or textbook?

Once you have a model that includes a universe and the capacity to simulate universes you can add universes to the model without taking much more complexity because the model can be recursively defined.

This only follows if your universe can not only model other universes but can easily model universes that share its own rules of physics. This is a much stronger claim about the nature of a universe (for example, it seems likely that this is not true about our universe.)