I never played 5th Edition and my grasp of those rules is pretty much non-existent. However, for 6th Edition, here's a couple of options from least complex (or at least, requiring less core rules changes) to most.
1. Every X rating of Armor adds +1 dice to damage resistance dice pool
This is basically bone density/bone lacing on steroids. Let's do the Min/Max Math(TM):
If I wanted to go all out, I could get six obvious "limbs" worth of armor; skull, torso, arms, and legs. That's 4+10+(2*15)+(2*20) capacity, for a total of 84 points of armor at a staggering cost of 420k nuyen. If every 4 points of armor gave us +1 dice pool, this character would have 21 extra damage resist dice, averaging 7 hits to reduce damage before adding body. The trade-off is that they would be virtually useless with Agility and Strength 2 for all actions using the cyberlimbs.
A less extreme example would be a samurai with 4 points of armor in the torso and each arm and leg, for a total of 20. Their Defense Rating would almost always grant them a point of attack, and they would get 5 extra dice to resist damage. However, this comes at the cost of 90k+100k nuyen just for the limbs and armor enhancements, so it doesn't really scale well when considering that Titanium Bone Lacing is 30k nuyen and 1.5 essence for +2 Armour and +2 soak dice, while Bone Density Augmentation R4 is 20k and 1.2 Essence for +4 soak dice but no additional armor. You could play around with every 2 or 3 points adding to armor, but that just means the DM can throw virtually indestructible NPCs at you, as the game mechanics would just break.
This brings us to...
2. Every Rating of Armor adds +1 to Armor and +1 dice to damage resistance, but Armor enhancements are capped at X
By capping how much armor can be added you fix the exponential issue with 84 armor enhancement NPCs, while improving the cost/benefit ratio over cyberlimbs vs bone augs. For example, if armor is capped at 1 you could add +6 dice and +6 armor for 30k if you armored all six limbs (skull, torso, arms, and legs). You still have to spend 90k on all the limbs, though, but you now have a significant boost in both armor and soak without the possible extremes.
If you instead cap at 2 per limb you're looking at +12 armor (which is already quite possible under CRB rules) and +12 soak dice, for 90k + 60k. 12 soak dice has a probability of adding 4 hits to your soak roll, meaning your durability would be significantly increased compared to nearly every other archetype, but at a relatively steep investment cost.
However, if you think this overcorrects, you could instead combine the two approaches and go for...
3. Every Rating of Armor increases Defense Rating by 1, and Rating / 2 (rounded up) adds a dice to damage resistance tests; armor attribute enhancements are capped at 3 per limb
This approach yields the same maximum result of +12 dice to soak, but increases armor by 18 (again, this latter part is completely feasible under core rules). It now costs 90k+120k to gain the maximum advantage of armored cyberlimbs, and you're extremely likely to gain edge on defense tests. I personally think this is a reasonable compromise between cost and benefit.
There is of course another approach...
4. Armor enhancements are capped at 1 per limb, cost is increased to X per enhancements, but now provide Hardened Armor
Limiting to 1 per limb is crucial, I think; with a skull, torso, two arms and two legs up-armored you would auto-soak 6 damage, making you impervious to most small arms fire, and any modified DV less than 6 would do 0 damage. This is a DRAMATIC increase in survivability, as it would basically take explosives or extremely high-net hit assault cannon shots to damage you, so the cost should be equally steep. Maybe make it... 50k per enhancement, for a total of 300k if you wanted the full suite? Adjust to what you think is fair, as long as you realize this can seriously destabilize combat.