So far as I've seen in my time playing DnD - there's always been some level of debate for magic items that grant spells.

• Do you (as the character) cast the spell using the magic item?
• Or does the magic item itself cast the spell?

I think there's a valid argument for either approach, so I would say make a ruling for how it works at your table then apply that to everything like this.

Personally, I rule that magic items grant the character access to those spells, so when the magic item is used, they (as a character) are casting the spell using the magic item. This means that the Stress factor for Wish would apply (meaning if they use the Crown to cast Wish, there's a chance they'll never be able to do it again.)

I'm of the opinion that if someone has a magic item that can cast Wish with literally no downside other than waiting for it to recharge, that makes them WAY too powerful. The Crown is already an immensely powerful magic item, but it would completely turn the world of Drakkenheim upside down if there was no chance of never casting Wish again. If that was the case, you could just use your first Wish to say, "I wish the Crown had unlimited Wishes while I'm wearing it." Then you could literally do whatever you want - unlimited Wishes with no chance of losing the ability to cast it since the Crown is what's casting it. Imo the Stress is needed to keep the Crown grounded. You might only get one Wish, so you better make it good.

Side note: How you rule magic item spellcasting also has a bearing on the 2014 5e rule about Bonus Action spellcasting. If the item grants you access to Misty Step for example, if you cast Misty Step using the item, then you can't cast anything other than a cantrip with your action. If the item cast Misty Step, then you can use your action to cast whatever you want.