Maxed out for it's task is fine. It's the percentages that are of. The percentage of available potential increase in defensive ability. There are to many modules that can be fitted, to increase the amount of defensive hit points. All these stack and most can be engineered. Nothing else in the game can be compared to that.
If a pure trader should have the same options, we would have to have cargo racks that were engineerable for both size and efficiency. On top of that, we would need an external cargo bay and cargo booster modules.
Explorers can fit one range extender. HRPs, MRPs and SCBs can be stacked.
If you go for defense, you simply get more. The modules are also poorly balanced between each other. HRPs generally give better protection/ton than armour. Shield boosters have no weight or power penalty for bigger shield generators.
The weight penalty on internal defense is to small and the speed penalty on weight is to small. Even on a pure combat ship there should be a balancing act between speed and tankyness.
Engineering benefits everyone, not just combat.
Explorers benefit most from FSD mods. So do traders.
In fact these 2 roles benefit the most from FSD mods because they tend to run light. Of course the freighters benefit less because of their smaller FSD modules.
Besides FSD mods, the various scanner mods for wider scan angle or longer range, all those also count for exploration tools, do they not?
Fighters benefit the least because most combat oriented ships have even smaller drive modules. And don't forget, most combat roles have reinforced or mil bulkheads which add A LOT to mass, and further decrease jump range. Stacking HRPs, MRPs and SCBs also add to mass.
Unfortunately, traders perhaps, get the shorter end of the stick, because there is not much you can do about physical cargo space.
The only thing they can do is to decide between going all out for the FSD mods so that they can out-HighWake an attacker, Engine mod for realspace speed, or tank up with a Mil bulkhead so that they can buy time tanking attacks on their way out of Dodge. They can also choose to stack SBs.
Engineering is not the problem. The basic ship design probably is. Base hull values, and module configurations.
Then again, if you put a 747 against an F-15, you already know what's gonna happen.
That's just a reality you have to face.