Ironically, the way the game is designed, it actually doesn't really encourage "playing the map" with respect to late-game strategics. If you know that you'll need plenty of aluminum later in the game, the only reasonable strategy is to blindly expand, as you said. This isn't a map-dependent decision. I think there should be a way to set up outposts (much more lightweight than cities) on unclaimed tiles, which will actually allow players to make more dynamic decisions when it comes to resource acquisitions.
With regards to making iron more common, I don't think this would work with the game currently is, since it'd mean more disruption to district planning. For me, prohibiting district placements on revealed strategics is, hands down, the worst decision the devs made in Civ 6. There are many features in the game I don't like, but most of them, I can at least understand why they're there or I can't really easily think of better alternatives. This restriction is just so obviously problematic because it makes players want to avoid unlocking certain techs because they risk getting (rather severely) punished by the game for doing what the game should be encouraging them to do.