In light of keeping everything simple, why not just say:
Light armor gives a +1 AC with a max of +4 Dex.
Medium armor gives a +4 AC with a max of +2 Dex. Disadvantage on Acrobatics.
Heavy armor gives a +8 AC with a max of +0 Dex. Disadvantage on Acrobatics and Stealth.
Add in shields, as usual.
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If you're trying to keep it simple, I don't see the point in differentiating between types of light and heavy and whatnot. Just give bonuses on the type, let the player and DM color in the specifics.
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Then special materials like mithral and such can make a difference to the base stats.
And magic, of course.