The described problem is one any int hero faces basically.
The solution is an item that actually pierces spell immunity on all spells. (maybe 60% spelldamage is converted to superior magic damage and maybe Grimore is a good item for such thing).
For magmus, there are quite some nice ideas, I'm still in favor of making him able to move while in sandstorm. I wouldn't directly buff his late game scaling with SOTM as this is a global problem and it requires a global solution imho.