It might just be the 1.18.2 bug where armors have no stun resistance, but it seems incredibly punishing to use a greatsword when compared to any other weapon, for one simple reason: your attacks are cancelled by any hits!
The player will already be taking a lot of damage thanks to the slow speed of the weapon, it doesn't seem too unbalanced to allow swinging the heavy weapon around without getting interrupted so easily.

Will be added as a passive skill :) great idea tho.