How I would do it.
Knockback attack. If successful replace the normal weapon damage with your STR and roll your damage modifier as normal. This attack does not cause damage but, if the result is higher than the opponent's, SIZ then the opponent is knocked back as per the normal rules. A knockback attack can be parried or dodged as normal. (Parrying a knockback attack allows you to deflect some of the force of the blow.) A knockback attack can be made using an unarmed skill or a weapon skill. Long impaling weapons such as spears or flexible weapons such as whips and flails can't be used for knockback attacks - unarmed skill must be used instead.
Shield Bash. This is a knockback attack as above. Unlike normal shield attacks, this one does not suffer a -20% modifier. (Shields are good at bashing.)
I like this even better. Thanks for posting.
There is a risk of this being overpowered because a shield user could use their bonus shield action to get an additional attack which the target may not be able to defend due to being out of reactions by this point.