Smash! Smash! Smash!
GMBinder: https://www.gmbinder.com/share/-OmkKlsSB2zwL46gJt7I
The Machine-Breaker is a tricky one. Given the theme of Machines of Bone & Blood, it is required; the most iconic image of a Luddite is a burly man taking a giant sledgehammer to a factory full of machines. But it's actually a tough concept to get across in D&D, because highly-specific subclasses usually come out bad. Hence, this one needing two drafts before it reached a spot where it's shareable on here.
Smashing Blows at 3rd is your core feature. Once per turn, you can deliver an especially powerful melee attack, which deals bonus damage, grants an ally advantage, and disables one of the target's damage resistances. That bonus damage gets doubled against constructs. Against the Machines is a flavor feature, giving you a boost when figuring out machines. At 6th, you get Tools of the Enemy, which lets you take any damage resistance you disable, and Staggering Smash, which makes it tough for the enemy to attack the target they want to. Righteous Hammering at 10th lets your allies use Staggering Smash. And Have You Tried Kicking It? gives you a unique way to make use of unknown machines. Rebellious Leader at 14th gives your allies a boot when you use your Smashing Blows. And Strip Defenses augments Smashing Blows to be able to downgrade immunities, providing an answer for a common late-game challenge.
The whole disabling-resistances mechanic is pretty gamey, to be honest, and can produce absurd results; are you really going to make the fire elemental not immune to fire? But I don't think it's too much. Most things with a mere resistance (as opposed to an immunity), the idea of taking that away doesn't break immersion. And the idea in my head was a barbarian who hits so hard they tear away the enemy's defenses, but implementing that via AC is too bitty, via extra damage is too boring, and via advantage is too flattening (trivializing all the other ways the party might be getting advantage).
As always, you're invited to come discuss and offer feedback in the comments or on Discord!