Therefore:
• High tier Martials can have class design space to CHOOSE a magic item.
It is a bit iffy to explain HOW nonmagical characters can guarantee a magic item. Maybe call it "fate"? Maybe the magic item somehow finds them, for some guaranteed reason? The explanations needs work to sound plausible for a nonmagical character.
But in principle, it is ok for a Martial to do powerful magic, by means of a magic item.
I also think this is why, making "spells" and "rituals" two different design spaces, and allowing Martials to try perform rituals solves many qualitative differences between Martials and full casters.