That also isn't a bad idea, tying them into the tiers for the class you gained the style in.
The only suggestion I have would be to remove the stealth thing (since most warrior-types with armor won't be trying stealth moves, at least IMO) and maybe if you house-rule the disadvantage for wearing armor applies to Athletics and such, the better Defense style could remove that? Just a thought.
Parties do group Stealth checks enough in my games that even though armor-wearers aren't supposed to be "good" at sneaking, making Stealth checks is something they still have to do. And since the disad on Stealth is already tied into the armor chart, it's an easier award to give than having to create a penalty of disad for Athletics checks first for the entire game, only to then remove it for the second tier of Defense style.
Like using part of the Dual-Wielder feat as a second tier award for the fighting style... the removal of disad for stealth is using part of the Medium Armor Master feat as a second tier award too. You were questioning what was more important-- fighting styles or feats-- and for my money they both have aspects that are very important for certain playstyles. And I'd rather encourage those playstyles by giving the reward away for "free" (if you have the fighting style), rather than force someone to take a feat that includes it (but which might not be worthwhile overall.) No one in any of my games ever has or will take Medium Armor Master (because there are usually much better options for people), so I just take out one of the bonuses from it and use it elsewhere. But to each their own.