so the planet killer is basically a Space Monitor . Big guns , but very slow and vulnerable .
does Star Wars have an equivalent of a Space submarine ?
The Star Destroyer is one of those Japanese carrier / battleship hybrid .
Not as far as I know, but I have had thoughts along those lines before now.
Imagine this:
Your FTL technology is such that practical ships, i.e. ones with a useful payload fraction, have to make 'blind leap' jumps through hyperspace. The point where they are going to come out in normal space is determined by what they do at the start of the jump, and they are blind and powerless to change anything once in hyperspace. However, by devoting almost almost all it's mass to much more complicated and power-hungry equipment, a ship can 'hover' in hyperspace for a time, and the same equipment gives it both the ability to detect other ships during their jumps, and to project a disruption field into their path which causes their own hyperspace field to collapse, with the result that different bits of their ship re-enter normal space at different points along their path

Just as with a WWII submarine, the 'ambusher' ship has severe limitations as well as scary capabilities. It can't move very fast in hyperspace, so it has to move slowly in normal space, often to inconvenient locations, in order to 'hover' at a a suitable point in hyperspace (the mapping of normal space to hyperspace being, by definition, variable and non-linear). It's detection range in hyperspace is limited, and the act of projecting the disruption field causes it to pop out of hyperspace itself, with most of it's energy drained, and the possibility of this leading to it's own destruction (by re-entering in the middle of a star or in deep interstellar space, too far from anywhere to refuel/recharge) is a limitation on when it can fire without sacrificing itself. You could add more limitations. One thing that occurred to me was that perhaps control of the hovering/detecting/firing is only possible by telepathic and/or mutated individuals who are in short supply, thereby limiting the number of ambushers that could be deployed at any one time.
The character of a battle between normal ships and hyperspace 'bushwackers' might therefore take on the character of a WWII submarine/anti-submarine battle: a hidden enemy taking out helpless cargo ships with escorts using various semi-effective countermeasures while the respective captains try to out-think their unseen enemies using psychology and mind-games.