Personally I’m considering converting the expertise dice system to the boons system from Shadow of the Demon Lords:
Expertise dice are all d6s, and you can have as many of them as you are granted. When you roll your d20, you roll all your expertise dice (if any) alongside, and you take the highest value d6 rolled.
This keeps the bonus from expertise dice bounded as well as introducing a decreasing marginal benefit for stacking expertise dice. It is also a simpler concept to understand than having stacking expertise dice increase their sizes in steps.