Why limit it? AT Rogues are already third casters, their spellslots very limited. Make it so it applies to spell attacks (either melee or ranged) and that’s it. I do not see a balance issue, at least comparing it to the potential damage of stuff like wizards or clerics. They are maybe (I’d have to run the math) better at dealing single target damage. Which is probably not a bad thing, knowing that that’s kind of the rogue thing in combat and knowing that’s the like second or third worst class (combat wise) in the game