Good points, but I'd like to offer some counter arguments to the megascale engineering idea, hopefully without sounding too pedantic.
The need for megascale projects is not as obvious as it's sometimes made out to be. There might well be reasonable limits beyond which the maximizing of habitat space and energy consumption tapers off in utility. Even if our civilization continues on its present course, it's not certain that we will get to the point where we (or some of us) feel the need to change the face of the galaxy. If you go one step down from the galactic stage, the detectability of solar system-scale megatech is also open to debate, especially as you move beyond a couple hundred light years from here.
You're right of course when you say all it takes is one civilization (or even one member of a sufficiently advanced society) to undertake projects that are liable to make all kinds of strange phenomena appear in our night sky. But the number if beings capable of doing so on a whim, multiplied by the chance they'd be interested in such things, might be very very small without breaking any of our current expectations about biology and physics.
Finally, I'd like to address the simulation argument. There are two extreme ends of the simulation scenario spectrum.
At the one extreme, we're talking about an artificial reality made especially for us, humans. This would be the popular "Matrix"-style simulation. No scientific argument I've heard so far makes it a convincing model for the reality we live in, except as a possible explanation for the Fermi paradox.
At the other end of the spectrum, we have a full simulated universe. It's hard to argue for or against that notion, simply because either way it fits with our observable reality. The catch is of course that it's a neutral proposition with no impact on us at all. This variant does not offer any explanation for the Fermi paradox.
As I see it, the simulation argument as pertinent to the Fermi problem offers two options, neither of which are intellectually appealing: one is a borderline-untenable just-so make-believe world where all rules are an illusion and anything can happen, and the other one is virtually undistinguishable from our "normal" world, up to and including an absence of any obvious explanation for the Fermi paradox.