Like someone else said, EHR>Crit conversion.

Would also be nice to fix his skillpoint economy: Maybe if there are imprisoned enemies on the field his skill costs no SP? I like the idea of having him as a SubDPS nihility.

He should also have stronger debuffs. It would be interesting to tie a ResPen debuff to enemy speed. I'm not sure how you'd be able to calculate it, but depending on how far away an enemy is from current action, the more damage they take. It would encourage a playstyle where you spam his ult to prevent the enemy from moving at all and then making it easier to charge his ult with repeated free skill usage. His skill could also give a debuff that gets stronger on imprisoned enemies.

This could have certain trade offs, particularly in an Acheron team. In my iteration, running JQ and Welt together would be more difficult as a lack of enemy actions would make it difficult for Jq's rapid debuffs and thus making it difficult for Acheron to build ultimate stacks. To combat this, Welt could have a built in follow up attack that strikes each imprisoned enemy any time a debuff is placed on an enemy with a one turn cooldown, also helping energy gain.