Remarkably swift! Expertise and Pierce Parry are both great--I was actually just experimenting with setting up flails and Pierce Shield so that they're harder to block, so having a source of parry piercing as well is fantastic!
My only remaining wishlist element then is not having an avenue for a scaling expertise formula, simply for concern over how to balance expertise against scaling enemy parry chance. I'm going to need to do some brainstorming over how to best present expertise equipment / skills / actor perks to players in a fashion they can appreciate defeating enemies with high parry (or really, a way to visually ID parry-happy enemies in general.) Maybe a state icon, or a 'parry aura' effect as visual shorthand. Hmm.
I realize this is ultimately a sliding scale issue to begin with though, since it will be difficult for a player in an RPG Maker game to really get a transparent understanding of their odds / the 'why' of how these calculations occur in a given battle. That stems from the broader boundary of RPG Maker MV's UI limitations in general though--my kingdom for the ability to mouseover a UI element and get a tooltip with information for the player!
Though writing that, I'm prompted with the notion of potentially having an optional animation for expertise making the difference for an otherwise parried hit (I imagine that is not actually viable, with expertise reducing parry rate instead of being its own calculation--but it's a fun mental image for one battler to do a parry 'clash' animation, only for the attacker to do their own expertise 'clash' animation to 'overrule' it.)
Much oblige for your efforts, stellar materials!