This, exactly this. Tanks have been in bikini armour since glamour prisms first became a thing. Just look at the original coerthan gear.
A healer or caster in plate isn't exactly immersion breaking, in fact its sensible and there's a precedent. If the devs are looking to ff4 for references, perhaps they chanced upon a rather key character called Golbez? A man who fights with top tier magic while in a suit of armour that would make a tank jealous
