I think a huge factor you don't account for here is that some of these technological improvements might imply a great-filter that we really haven't passed yet as humans, and the negative effects would affect most similarly expansionist and competitive races alike us since it might be questionable if there would be enough pressure on a non-competitive race to expand rather than just conserve local resources.
Just with state-controlled nuclear weapons we've been on the brink of extinction a couple of times already, the energy levels required for star-travel implies this kind of destructive power being in the hands of even more people (and by necessity more or less out of control of the nation states). A commercial airliner took down WTC, a starship would be an WMD capable of taking out a city (or more).
One implication of this is that there's a chance that we've already invented practical fusion power, but if it's trivial to miniaturize AND weaponize then people in control of it have decided to withhold it to avoid every weird terrorist group creating one.
You might want to check the physics on your assertion that a starship could take out a city. It’d have to be designed to do so otherwise it would just vaporize as it entered the atmosphere at the velocity you’d need for that kind of impact.
I haven't done any calculations (since we don't have any feasible crafts for interstellar travel that's irrelevant really), but considering it for a few minutes I'd say there's 3 increasingly likely ways around that.
1: Considering the amount of rocket fuel we need to leave earths atmosphere and reach the Moon, people have been proposing nuclear rockets to reach Mars. That's still within the solar system, reaching another star requires magnitudes more energy, even more so to accomplish enough acceleration to reach another star within a persons lifetime. Such a mode of energy generation not having an explosive failure more feels unlikely (thus making it blow up in a dock is enough).
2: Barring option 1, reaching fractional light speeds, would not a ship need enormously more capable shields than anything today to safeguard humans? The Tunguska event(3-5 mt) was at "just" 27km/s of a 50 meter object.
3: Speaking of Tunguska, even if the ship itself would lack such shields (however a human would be expected to survive w/o one), a ship capable of interstellar travel should be able to push out a rock and then accelerate it back to earth to create a Tunguska (or larger) event at a target location.
The core issue is the energy levels required(1), converting them to something destructive is usually within grasp of less intelligent people than those that research the advances that make them available.
If we’re talking specifically about interstellar craft with enough shielding to survive an uncontrolled reentry at high velocity then what the heck are those going to be doing near a planet? Sublight travel would have to be performed by craft large enough to support the crew for years if not decades or generations. You’re not going to want to maneuver that much mass into orbit around a planet. They’d be better off parked in a trojan orbit and letting smaller craft move people and supplies back and forth You might as well try to hijack an aircraft carrier and fat chance of surprising anyone if you could pull it off.
To get a ship to hit the ground at the velocity you’re talking about a large chunk of it would need to be solid steel like a bullet basically. Space craft aren’t built like that, they need to be mostly empty space for storing propellant and people. A reactor and its shielding might survive but that’s on the scale of 5 - 10 meters and it’s still not 100% solid so it doesn’t compare to a large metallic asteroid.
Throwing rocks at a planet might work but you need the right equipment and expertise to bullseye a planet from 100 million miles away and if anyone saw you do it they could take their time intercepting the rock.
Just with state-controlled nuclear weapons we've been on the brink of extinction a couple of times already, the energy levels required for star-travel implies this kind of destructive power being in the hands of even more people (and by necessity more or less out of control of the nation states). A commercial airliner took down WTC, a starship would be an WMD capable of taking out a city (or more).
One implication of this is that there's a chance that we've already invented practical fusion power, but if it's trivial to miniaturize AND weaponize then people in control of it have decided to withhold it to avoid every weird terrorist group creating one.