Sometimes too many historical examples can confuse basic knowledge. Very many factors involved in each historical case IE: were those particular cavalrymen heroic, ready to martyr themselves to defeat a hated enemy? Were those troops irritated by poor weather and sour water rations? This is why dealing with the physics should be primary - physics don't change.
I think R2TR cavalry are in a very good place even though they will be modified more with time. I love to see cavalry charging in to infantry and dying in the process because I know those infantry are covered in metal and with sharp objects in hand. I like having to be very cautious about using my cavalry because it feels more realistic and it means I can focus my attention on very specific infantry formations. But, however,
Ariamanesh makes a lot of good points and maybe just a few of the best cataphract units need to be more capable of breaking heavy infantry.
The Gauls: I wonder if geography gave the Celts an advantage when invading Greek lands. The wide open space in the Pannonian plains becomes narrow and mountainous while travelling south. Greeks were a Mediterranean sea people who probably spent too much time building ships and not enough time fortifying their northern land borders. Celts had all that good land in the Balkans supporting them while they raided down the mountains in to Greek low lands.
Compare this to Italy where there is a wide open plain in the north which separates the Alps from Italy's most important cities. This area can be fortified/militarized while Balkan lands are much more complicated.