[Dragon of the void]

Can't you do a relatively cheap, fragile planet buster tech that only fits on a starbase ?

Well, yes, you could abuse this with a SYS, but if you control the sky enough to build a base on a hostile planet, you really own this place. And the (hostile) planet could just build any defence, since the base will start right next to the planet in combat, while attacking ships have to first get there ...

Maybe even give this starbase/selfdestruct-trigger the ability "destroyed after use" like when creating a ringworld ?