What do you mean by 3 tiers to 6? Both dex & strength builds get a dex bonus to AC in the table (up to +6 for leather& +1 for plate) but both dex & strength builds need a point or two with of mods or reduction of the other if they want to max out AC. It also makes it so the GM can have another way that could allow them to suit a bit of armor more towards Alice than Bob.I really don't like scaling the dexterity mod. I don't like it on the current rules TBH, but going from 3 tiers to 6 would be worse.
I think all characters should get full dex mod to AC. I also think encumberance and a movement penalty is the best way to deal with armor weight and better than making it solely based on strength.
Movement penalties do good things for tactical grid combat & incentivizing mounts, but they often tend to just penalize heavy armor & give dex builds a free pass despite the same or similar ac. A system where both strength & dex builds need a bit of the other for the best ac could mitigate that problem by needing slightly more strength for dex builds thsn dex for dex builds or some other secondary benefit for strength builds