Some thoughts about mechanics is that for AC, there could be a system of: Some class-based number like (8, 10 or 12) + Proficiency bonus for AC.
Armor could be changed to Temp HP, either a flat number like 5 or 20, or a multiplier to proficiency bonus like 3*Prof bonus. With some armors also including damage reduction or a small AC bonus (just +1 or +2).
For something between the Industrial Revolution and WWI, keep in mind that Breastplate was still an armour that was used. I remember something on how the logistics for Napolean's army was like how Breastplate was good for stopping one gunshot.
Heavy armor training could be something that's removed from all classes, and only applicable through feats or special subclasses (if going the Steampunk route) such as a Mechanized Juggernaut Fighter subclass.
Since it's before WWI, I'd make sure automatic weapons (though remember the established rules for automatic fire is an area attack with a dexterity save to avoid all damage) are excluded, and only something available as siege weapons (since gatling guns in the 1800s needed to be pulled by horses) or large sized monsters like an Ogre Heavy Gunner. Maybe PCs could be allowed some, but only if they take a special subclass like a Heavy Gunner Fighter subclass where they'd be like Jinx from Arcane/League of Legends except more strength-based.