I think a linear progression makes some sense in gameplay terms, and there are plenty of arguments for this in design terms, as opposed to just the consideration of pure material properties.
e.g. say copper is ~20% more dense than iron, that just means you use 20% more iron to make the same weight ...
There would also be strong arguments to say the superior rigidity and hardness of iron would far outweigh the density benefits of copper in creating a weapon design useful for fighting.
Just some thoughts, anyway.
Indeed- it works well in Dwarf Fortress because the gameplay emphasis is on crafting and preparation, not tactics. Thus, it behooves the player to learn about the different material properties to create a variety of interesting and effective weapon/material combinations, instead of just spamming Ubermetal Weapons.
In a game where you're more worried about the actual fighting than the making of the weapon, a linear progression is an acceptable break from reality.
I am wary of avoiding any 'simplification', but I do think as monk12 says that the gameplay emphasis isn't on crafting/construction, and therefore I don't want to make learning about material properties unnecessarily complex. However, I might add in requirements for different volumes of different metals to create the same weapons, and different crafting requirements, but in terms of damage/combat, I think the progression will remain linear.
A few slightly more serious questions than my last one:
I've tried reading through as much of this thread as possible, but it's hard to keep up, so I apologize if this was asked before. Have you had any thoughts on traps like pitfall traps, spike pits, swinging log ram things, and so on?
Also, since this is a strategic game, terrain is going to do a good bit of tactic dictating. But would these have any numerical value? (For example, an enemy charging up a hill would get fatigued faster and hit the defenses with less force, or a force wading through a stream being much easier to hit than a similar force moving through a field)
My questions might be worded poorly, but it's just that once I get started thinking about this game, my mind wanders off and I lose my train of thought.
No problem

. I have considered traps, and they're certainly a possibility, but a long-term one. I think I'd be most inclined to put them in for cities than camps, but I think it would be nice if you could quickly throw up some basic traps.
Yep, moving up a hill will slow you if you enter combat immediately after, and going up a z level will take more stamina to do so. Wading through a stream has a chance of your movement being unsuccessful each turn. I'm hoping to also add effects based on snow, sand, weather conditions, etc. Similarly, you can see any z level below you but not the ones above, so that's a huge incentive to reach the high ground by default (both for you, and for the AI).
also, would having archers attack from a hill give them a range bonus, and archers attacking up a hill have the opposite?
Yes for attacking from a hill heading down (since I'm actually going to the effort to model projectiles in an 'arc', so aiming at something lower will get it 'further' than aiming at the same level), not sure about fighting upwards, since targets will likely be out of sight. I'll probably just leave it to whatever projectile function I dream up!