I like all of this table; I'm just not sure I see the need to have a scaling feature linked to proficiency bonus in it. Unless there's a belief that AC should be scaling in base 5e, of course.
Low level characters have inferior armor. High level don't. That is 1 point of AC scaling.
Light armor PCs in dex usually gain +2 AC from level 1, so a +3 swing.
Medium armor PCs gain 0 to 1 AC, and 1-2 from armor quality, so a +2-3 swing.
Heavy armor PCs gain 2-4 AC from armor quality.
The proficiency calculation there is on the same order of magnitude as the existing swings. The stats I chose (except heavy armor) are low enough that a level 1 PC can probably hit them. Heavy armors 15 str/con is intended as a neat stretch, and is the best AC for a low-dex PC.
But sure, I can strip out the proficiency bonus.
Light Armor: 12 AC, +4 AC if 15 or higher Dexterity.
Medium Armor: 14 AC, +1 AC for each of 13 Strength or Dexterity
Bulky Medium Armor: 15 AC, +1 AC for each of 13 Strength or Dexterity (disadvantage on stealth)
Heavy Armor: 16 AC, +1 AC for each of 15 Strength or Constitution (disadvantage on stealth)
"Superior" versions of each armor is worth +1 AC.
Superior Leather is studded
Superior Medium is a breastplate
Superior Bulky Medium is half-plate
Superior Heavy is full plate
Baseline at level 1:
12 AC in light
14 AC in medium
15 AC in bulky medium
16 AC in heavy
this ensures that "heavier armor" for a class is actually a feature. Also, if you don't invest in physical attributes, medium is better than light.
Optimal:
15 Dex: 17 AC in superior light
13 Dex/13 Str: 17 AC in superior medium
Disadvantage Stealth:
13 Dex/13 Str: 18 AC in superior medium
15 Str/15 Con: 19 AC in superior heavy
also, heavier armor gives more AC.
If you invest in dexterity and care about stealth, medium armor isn't useful, as intended.
Losing stealth gives you 1 point of AC, and heavy armor gives you another. Getting the bonus AC from heavy armor requires more physical training.
A 15 strength/10 con/10 dex PC will get more protection from heavy armor than from medium, however.
At 15 strength/13 dex/10 con, medium matches heavy. You lack the endurance to wear heavy enough armor, and your dexterity is best suited to less weight to move around better.
At 15 strength/15 con heavy wins, regardless of dexterity.