small change to make heavy armor mastery feat viable at higher levels

[Malifice]

It's now a scaling feat, and who doesn't like things that scale as they get stronger?
And honestly, by the time you're getting the DR of 5 or 6, it won't feel super strong. Sure, you can shrug off the adds a bit more easily, but the big bad is still gonna put the hurt on ya.
But, being able to sorta ignore the small mobs around him is what a Tank is supposed to do, right?

I'd consider picking up HAM if it scaled with your Prof Bonus. And worked on all PBS damage, instead of just non-magical.

After you've been hit 7 times between long rests at 20th level (saved 42 points of damage), you're ahead of a PC that got the Tough feat (+40 HP).

So after like 3-4 rounds of combat youre ahead, and it gets better from there.

And thats before the +1 Strength kicks in.