but time is relative. we might very well live in a simulation that takes a minute of “external time” to compute a single tick of our time. we just can’t experience it.

I'm not really a believer of the whole simulated universe theory, but I find your arguments against it weak.

You're basing what is and isn't easily calculatable off of our experiences. Same with "complexities of the universes". However, if our world is indeed simulated, there's no telling what the host universe is like. It might have crazy different math and be far far more complex than ours. Us trying to understand it would essentially be an excercise in futility.

there would be more sim-universes than real ones

This ties back to the mediocrity principle. If there are 10 billion people living on Earth, but 10 quadrillion living in simulations, the chances for you to live in the latter is much higher.

Along goes the simulation argument by Nick Bostrom. If simulation is possible, and practiced, we likely are simulated ourselves.

Isaac Arthur) noted that housing a population in a simulation is much more efficient than doing so physically. It seems like a convergent choice for powerful civilizations which want to maximize the life supported by fading stars (or energy potentials in general).

I think it would take the computing power of an entire universe to simulate one of similar complexity

Two objections:

1. It might be sufficient to simulate the experience, without fully simulating the underlying physics. That's how we do 3D games anyways. No one cares if we actually simulate individual air molecules. If the cloth moves indistinguishable as if, that's as good as the original, for a much lower cost. You can also cull unobserved parts of the universe.
2. Host and simulation can have completely unrelated laws of nature. Specifically, inhabitants of the simulation cannot study their host environment. As such, I think making assumptions about the host makes no sense.