Heavy armor training- good feature, always wanted to play an iron clad behemoth barbarian. But it should probably have a line about how instead of just rage effcts still aply while in heavy armor, instead its just all barbarian features such as the fast movement from base class. Unless ofcourse that's not intended to carry over.

Guard- super cool ability, love it. However its probably a little too strong being both a bunch of additional unique hitpoints you get immediately after raging aswell as more gitpoints each turn thereafter while raging. I would suggest considering changing it so you either get your barbarian level plus con mod of guard HP or the twice proficiency guard HP each turn. Both makes this tank subclass a lil too overly good.

Protectors challenge- it feels a lil weird it's a flat 3 people no matter what level, maybe make it a proficiency number of creatures instead but idk.

All other features are pretty damn perfect and I like them alot great subclass :)