Text version:
Magic-averse Staff (Wondrous item)
Weapon (Quarterstaff), Rare (requires attunement)
This weapon, with its unassuming appearance, stands as a silent testament to resilience and serves as a steadfast companion for those who choose to tread less travelled paths.
- Weightless. This weapon has the Light and Finesse property, as the wood it's carved from is very sturdy, but also unusually light.
- Pierce Magic. Attacks made with this weapon bypass the Armor Class bonus derived from magical sources, such as magical armour or the Shield spell.
- Arcane Scourge. If you hit a celestial, a fiend, or a creature that has the Spellcasting trait with this weapon, you roll one additional damage die.
- Spell Breaker. A creature hit by this weapon has disadvantage on Constitution saving throws that it makes to maintain its concentration on a spell.
- Fizzle Magic. The staff has three charges. While wielding this staff, you can expend one charge to cast Counterspell (only if you are one of the targets of the spell, or you are included in the area of effect of the spell) or two charges to cast Antimagic Field (no material components needed). Charisma is your spellcasting ability for these spells. The staff regains all charges daily at dusk.
Cursed. While attuned to this item, you cannot cast spells or be attuned to other magical items. If this curse is broken, the wood from which this quarterstaff is carved rots away, resulting in the destruction of the item.
TBF, the +2 armor not only is magical per canon, but even the sub-question is answered in the ability summary: "Attacks made with this weapon bypass the Armor Class bonus derived from magical sources, such as magical armour or the Shield spell" ๐ค So, yes and yes.
I say this first, because I really like your second point, when applied to this concept and would actually love to see it appended to it. "If the target is concentrating on, or the target of a conservation spell, add Prof to damage dealt.", or similar. Really ~~hammer~~ shillelagh the point home that it's a wushu mage bane legend. ๐ค๐ฝ
Lastly, you might even alter the third point to be truly silly and grant an AoO even if the wielder doesn't have a reaction to spend, but it causes Disadvantage on the PC's next use of the item.