I think others wouldn’t bother with us until we started demonstrating likelihood of using dangerous tech or crazy exponential expansion.

Why do you think that, though? It doesn't make sense, frankly - if you're worried about competition evolving you shouldn't wait until the last possible second to destroy it. That raises so many unnecessary risks of being slightly slow on the draw, and then it's too late. Why not do it at the earliest convenience, when it's super easy to do by comparison and there's an incredibly long margin of error if you somehow miss the first couple of tries?

I don’t remember well, but I think civilizations stationed their defensive or offensive tech away from their own civilizations, just dispersed around.

I think its explanation for why no one or anything has colonized the galaxy though is that if anyone shows signs of becoming that strong, they get zapped.

But they're already doing it, you just said they're putting outposts out there. If they can't do that secretly then the Dark Forest doesn't work in the first place. Placing a secret weapon base in another solar system is no different from placing a colony there.

My real preferred theory of why we don’t see other civilizations though is that I think they choose more inward, VR, computer-based evolution that doesn’t result in big mega structures.

As with many Fermi paradox solutions this one fails on account of requiring every single civilization (and every single subset of those civilizations) to all decide to do exactly the same thing, forever, with no exceptions. In a scenario like this what happens if a single subculture of a single advanced civilization decides for whatever reason that they prefer not to do that? They would be able to spread throughout the cosmos without opposition, everyone else is locked in their little dream boxes and therefore is basically irrelevant. It only needs to happen once, and the universe has been around for a very long time.

Relativistic missiles. Nothing moves faster than the speed of light. So if you can get a big rock to go 95% of the speed of light, we'd only be able to detect that it's coming right as it hits.

This is a very common answer to "how", but it comes with lots of problems in the Dark Forest context.

• If you actually calculate how much energy is required to boost a big rock up to that speed you run into lots of difficulties. It takes a lot, a heck of a lot. How does a civilization that is "hiding" accumulate that energy? How does it store it long-term?
• How is that energy actually put into the rock? This is basically a starship accelerating up to that speed and getting a starship up to that velocity is not easy even if you have the energy available. Does it have a rocket? The rocket equation for getting up to near-lightspeed requires ridiculous amounts of propellant. Is it beam-propelled? You're not being at all stealthy that way. How much acceleration can you get out of your system? It takes a full year at one Earth gravity of acceleration to get up near lightspeed, and that's a really high acceleration - you generally trade acceleration for efficiency so the faster you want to get up to speed the more energy you need and the noisier you'll be.
• It actually is possible to counter an RKV. It's much easier to hit and destroy an RKV than it is to launch it, all you need to do is get a pebble in its path. The key is detection, and the above points give some pretty good options for detecting it before and during launch. That gives you time to fire your countermeasures.

And ESPECIALLY if your civilization is still mostly planetbound.

Absolutely not guaranteed to be the case. Earth's civilization could have easily had offworld colonies by now if circumstances had been slightly different, so a Fermi paradox solution that requires reliably blowing up Earthlike civilizations before they can get offworld doesn't work. They're already too late.

As I said previously, Earth has been quite obviously life-bearing for at least 2 billion years. Why wait until something like an RKV is needed, and even that is not guaranteed? They could have destroyed life on Earth far easier, and thus far more stealthily, if they'd done it a billion years ago.

But that's not actually true. We've been "broadcasting" the fact that there's life on Earth in the form of the spectrographic signature of an oxygen-rich atmosphere, which is a clear sign that photosynthesis is going on. There's no geological process that could maintain that much oxygen in the atmosphere. The Great Oxidation Event is when that started.

We have the technology to detect this kind of thing already, at our current level. Any civilization that could reach out and attack another solar system would be able to very easily see it.