We could not detect ourselves even in a nearby solar system. And as we've gotten more developed, we tend to be MORE frugal with our energy usage.
If you could cryosleep, lived for millions of years, or were an AI that could just turn itself off for a while, you could go to the stars using simple chemical propulsion. No physicists’ nightmare torch rockets or warp drives required.
The crazy physics requirement is to enable interstellar flight within natural human life spans. It’s probably easier to extend that or figure out how to hibernate or become AI than it is to build an antimatter rocket or bend space time (if that is even possible).
I doubt the dark forest because Earth has been broadcasting that it’s a likely biosphere via its albedo absorption spectrum for over a billion years. If you are a hyper paranoid alien you should just blast every biosphere to kill any possible rival before it even evolves.
Technological change is probably a sigmoid like most things. There could be people out there who have already won physics, figured out the entire unified theory of how everything works, and have developed most of the core technological stuff they ever will. At that point they might calm back down and become more stable over long periods of time. These are the kinds of civilizations that would send slow probes or one-way colony ships out and would be willing and able to wait.
It's very hard to think about potential aliens without dragging in anthropocentric or even recent-history-centric assumptions.
I don't buy the Dark Forest hypothesis at all, but it's also just not particularly a factor for us at our current stage of technological development, any more than a stone-age tribe has to worry about Mutually Assured Destruction via nuclear ICMBs.