Zardnaar
Deity
You're splitting hairs to confuse and minimize the original point. That point being that general era of the Star Wars universe has been recanonized and in that era they are using more or less the same technology they are using now. This flies in the face of the point that Star Wars shows steady technological advancement. They do not. Technology has been stagnate in Star Wars for at least 4000 years, likely longer.
As a side note: I find the current comparison between the GE and IoM to be a bit unfair. We are comparing the GE at the height of it's power versus the IoM while it is in a state of severe decline. To make an accurate comparison, we'd have to either use the IoM during the Great Crusade (when the God Emperor was still up and walking around) or the GE right before or during the Battle of Jakku.
IN that 4000 years technology did not stagnate. If you time warped a KoToR ship to the time of the Empire it would get chewed apart by modern weaponry and hyperdrives are faster now than during the KoToR times. For example just a few hundred years earlier lightsabers still had power cords.
Also KoToR still had more advanced stuff than the Dark Age of Technology which is perhaps (at best) comparable to the Golden Age of the Sith era in Star Wars which was around 5000 years before ANH.
In the old cannon technology did kind of stagnate. Improvements did happen of course but it was more gradual but that changed a lot from around the prequal era. From the Clone Wars through to Last Jedi you can see the changes.
Warhammer 40k has actually regressed. The First order has actually teched up faster than the old Imperial Remnant has in legends as they lacked anything like a Resurgent Class Star Destroyer which is similar perhaps the ships from the old legacy era (140 years after the original trilogy). Its why I said I would mostly ignore the old legends canon except where it doesn't contradict the new canon or you can use it for an example of how things work (in general) in the SW galaxy. Both the New Republic and Imperial Remnant i the old lore went smaller with micro technology and more droids to reduce crew sizes and improve firepower with the new Imperial ships being similar in size to Victory class Star Destroyers but with a bit more firepower than an ISD II (kind of similar to todays frigates having missiles than can take out a bigger WW2 era battleship).
SHips in Star Wars used to have things like the Warhammer ones but they do not use kinetic based artiller and old missiles any more (because they do not work). IN ESB for example they take Vaders fleet into an asteroid field and the shields bounce large asteroids off them for a bit, in the old EU an ISD was caught on the edge of a Supernova and survived although it fried all the sensors. The reason the rebels can blow them up so easy is plot armor and the power level of things like Star Wars proton torpedoes which are energy based shaped charges that channel several megatons to giga tons worth of power to break a shield (such as the bomber in Last Jedi). Its why Jabbas goons in RotJ al crap themselves when Leia as a bounty hunter pulls out a thermal detonator its basically a hand held nuke using baradium.
In the old legends material they do find some worlds with nuclear technology for example and they are launching nuclear rockets at each other and they were regarded as little threat either because the rockets would get shot down or the shields would protect them (implied they could take a nuclear explosion).
Its why I said in the OP its possible the Imperium ships could barely scratch an ISD if their guns are all but useless. Hell even in the movies you see a Corellion Corvette (a small 100 metre ship) take several turbolaser blasts from an ISD (which was not shooting to kill to be fair). Its not because the ISD weapons are weak but because of the strength of Star Wars shields, the rebels used an ion cannon at Hoth which is Star Wars tech for dealing with shields (and it was powered by an old reactor from an ISD type vessel in the old canon).