I second the idea of a metalworking guide. Or an x industry guide in general.
As annoying as slag could be, I think it would be neat if it could be expanded into a mini-industry of its own. A quick look on wikipedia suggests it could be used for glassware in addition to what's already on the plate, although being able to make potash already, this can happen with some work to make clear glass anyway. I feel that the slag to potash reaction could probably use a higher quantity payoff, however. You still have to make lye for the reaction, which requires ash, so you're really just putting together a 1 + 0.5 (ash) + 0.5 (ash to lye) to make 2. It's very inefficient, but it'd be less tedious to just make a large batch of ash, and turn those straight into potash on a 1:1.
Alternatively, batch jobs at the ashery would be a great way to solve that issue.
As for the question of whether to make steel more or less difficult to obtain, I feel that making large enough batches of steel to outfit your military is tedious on its own, at least in the early game. I suppose the bigger problem, if you wanted to throw balance into the equation, is that flux is such a common resource that by the time most people find iron ore, they have a way to go straight for steel.
The question, I feel, would be not how to balance steel on its own merits, but instead on how to balance its competitors. There aren't terribly many equipment-grade metals at or below iron quality; copper and silver are the only non-alloys of those, and even then silver can't be used for armor, while (Bismuth) Bronze is usually quite inaccessible, as tin is an often unusually rare metal to find.
I feel that you have made some good efforts in providing military access to alternate industries, however. Rock equipment is a very cheap, albeit cumbersome, alternative for metal-starved forts, and gem equipment appear to be (if dwarven preference in equipment choice means anything) a good competitor with steel. Leather upgrades are a good idea, but require you to kill things first, which requires equipment in the first place! That or a working livestock industry, which usually isn't a good option in the first couple years of a fortress, exception of the beasts that drove your wagon.
To bullet point this idea:
-Weaker than steel or weaker than iron materials would be used more often in early fortress defense or less important squads if they were more accessible. Tin's too rare to make bronze, and most maps have either copper or iron, but not often both, in my experience.
-Alternate industries are a good lead! Consider expanding that approach, while having weaker-than-iron materials easily accessible.
-Strong-as-steel and stronger-than-steel materials, short of the really strong stuff such as volcanic, bifrost, etc. should be within the player's reach, but rare enough to warrant being a prized possession. Iridium and (welded) mithril seem to be decent models of this to me.