Managed to complete the Iron Titan Area and found one more item for the main quest.
After some searching in the lowest floor, I found the feet of the prisoner and shrunk them down as well (after deactivating another anti-magic area). As a thank-you of sorts I got his iron shackles, which are probably the “Invincible Iron” mentioned in the riddle, though they aren’t called that in the game. Maybe I still need to “prepare” them in some way elsewhere. While clearing out the rest of the Iron Titan, I found a teleporter to a pack rat room, where, among other items, I got a new ranged weapon (a throwing blade that returns to your hand after a throw) with a magical effect (inflicts Bane’s Boiling Blood, so it might be pretty powerful, gonna have to test it out in the next area). After finding all the keys and clearing out the rest, I exited the area and leveled up my Slashing skill and Flesh magic (so better sword attacks and healing).
The next area appears to be a giant arm (with a hand attached to it) reaching into the sky… wonder what awaits me there.