iron is used everywhere
pistons in farms (wither, general, dungeon, melon), rails (for moving villiagers or items), buckets (used in vatallus, ice farms, cocoa farms, other farms), iron armor (which is most frequently used because of costs), shears (because jungle tree farming without them is very slow), anvils, and hoppers (automatic furnaces, smelting, sorting, delivering, transport, collection, and dispensing of items).
i don't plan on building on all of those sites, they're just what i'm currently considering. If at any time, 4 iron farms are loaded and i play for 3 hours on a day, 270 golems or 1080 iron should have been produced. Not ridiculous, but a decent amount for use