As already stated, it's equivalent in AC to the best +3 light armor you can get. I would have to double check to be sure, but I feel most (maybe all) +3 armors are already legendary just from that alone. So with a RotAM you're getting all that & a bag of chips without requiring armor proficiency. In that context it would easily be well past legendary.

Advantage to save vs all magical effects seems to me like it alone should also justify legendary. In my (admittedly limit) experience, magical stuff must account for around half the saves rolled by parties & myself.

+2 spell attack/dc is at least a rare effect if Rod of the Pact Keeper is any indication, but since it's more useful for a wizard, I would say on an item wizards can use it's at least very rare from that alone. There's also no reason it can't stack with both the Wand of the War Mage & Staff of the Magi at the same time.

Combine all that together & you've got what, in my humble opinion, must be an overpowered item compared to most other legendaries, or at least that's how I've been viewing it.