I get where you are coming from but I find that - and the other answers brought up here - to be a perfectly reasonable solution. But it doesn’t really seem to fit for me.
Otherwise, why call it out specifically for elves, gnomes, and zaharans and not for say, Nobirans or Thrassians?
Ultimately, this divide seems to be an ability related to inclusion in that race and it seems to change a fundamental divide in the game-thinking when you can simply add “I cast spells and wear armor” to any human class build. Admittedly, any human caster that can wear armor cannot have full mage casting, which is a huge disadvantage over the standard mage, but it still seems as if this advantage is inherent by the need to point it out for some races vs. others. This would lead me to believe, say, that a Thrassian spellcaster could NOT cast spells in any armor they were proficient with. Otherwise, why call it out as a benefit of those other races?