I don't have a fix for the bad AT's.
I make Essence and Channeling pure and hybrid users wear AT 2 if they want to cast spells above level 5. Also Robes are like a business suit in modern times, so spell casters get the attention from officials and such, like they should.
As for AT 5-8: There is a factual SLOUGH of armors that actually did nothing for your protection (Bezanted armor, knot armor, button stitch armor, rod and slat armor). And AT 8 is not so bad, just the penalties are worse.
Then there is the way armor works against certain materials. Although it is not modeled in RM Attack tables, but a padded or heavy cloth with leather coat clothing stops a mace better than a maille shirt. Fact is AT 5-8 was worn beneath the other ATs as a base.
Another fact of armoring is that cuir bouille is no defense against punctures, no matter how much padding you wore under it. (So AT 5-8 didn't either)
The metal types begin to address the puncture issues. Scale, maille and lamellar were fairly good at stopping most types of damage.
Maille was around already in 500 BC and plate even earlier: 1200 BC: Mycenean Panoply, but since weapons and armor had an arms race pattern of development, armor tried to stay ahead of the weapons of the time, then the weapons tried to catch up, resulting in reasonably balanced armor vs weapons.
Example:
The lorica hamata was a roman maille armor. It protected roman soldiers vs barbarian and roman weapons with some distinction, but against an 8th century viking, the lorica was probably inadequate. The viking's hauberk was stronger and could stop a broadsword, but not for certain. However the viking broadsword was probably warded off easily by 11th century Normandic maille worn with padding. The normans had even better swords than an early viking, able to do some damage on knights wearing their own armor.
I'm trying to illustrate that you don't need specialized tables per period and weapon type. A typical sword will cut your arm off if unarmored and will break bones beneath maille, even though it can't cut it. Coupled with minor advances (such as mantled riveting, baidana spring steel rings or even lamellae added to maille) maille will hold up against period weapons.