Maybe heavy armor doesn't allow spellcasting? I know I tried it with a Githyanki in medium armor (I stole Lae-Zel's amor) and I was able to cast spells using her armor and longsword.
As for getting to heavy proficiency, you just need to start as a race that has at least medium armor prof, then at level 4 you can select heavy, right?
I think not Heavy armor by itself. It is proficiency you need to cast.
Because my Human can't cast wearing MediumArmor too.
And you cant get Heavy Armor proficiency without having Medium one.
Dwarf it seems to be bad choice because he can have only 15 INT or Charisma in the beginning
