I'm working on a dnd feat for both 5e and 5.5e, but I've ran some troubling balance issues when mixing it with another homebrew subclass, I would really like for them to work together as they do the same thing, but when applied together they make cantrips as strong as 4th level damage spells.

The idea was from the Sorcerous burst feature, that allows for more dice to be rolled if you roll max damage on the Die, into both a both a feat and part of a subclass.

The subclass was
When you roll the maximum number on any damage die for a spell that deals fire damage, you may choose one creature damaged by that spell to take an additional die of fire damage of that type. (e.g., an extra d6 on burning hands, an extra d10 on fire bolt). This effect can only trigger once per turn.

And the feat was

You learn to make your fire wild, powerful, and erratic.

• Increase your Intelligence or Charisma score by 1, to a maximum of 20. 
  
  
• When you cast a cantrip that deals fire damage, if you roll the maximum number on a die for the spell, you can roll another die of its type, and add it to the damage. When you cast this spell, the maximum number of these dice you can add to the spell’s damage equals your proficiency bonus
  
  
  
  But if I have a character with both of these, It would make a firebolt go from 2d10 at fifth level to 4d10 if you get one ten roll. Is this too busted? If so can I fix it, or replace it to something similar that does the Erratic, wild, random, and chaotic feel? If so any help with that would be great.
  
  
  
  
  Thanks for reading, sorry for taking your time. And have a great day!