Exactly. We considered this as well, but as you pointed out, knocking a target away and at the same time debuffing it are pretty much two conflicting effects.
Have you considered to make this perk an independent additional skill? To unbound it from the shield bash skill and instead make it give a new skill for a shield user who has this skill aswell. This way it wouldn’t be necessary to knock the target back to do some damage and debuff or whatever else you see fit, but it would still be a utility skill with a utility use and it’s strenght would be limited to shield users only.
If you wish to bound it to reality more, well there were hard shield wielding practices to make a valid member of the phalanx who could not only hold the shield covering behind but to effectively use it to attack. Same goes for the more rare spiked shield users and other specifications.
