This ent looks an awful lot like Ivern

That's kinda because they are all designs within the treants archetype designs, just like how elves look very similar to one another across all titles with elves in it.

Maybe because Ivern is basically an ent, but overall they share no similarities is facial structure or hair.

I believe what you mean to say is the Ivern looks a lot like Treebeard. Haha

he's not underpowered enough

Its the other way around. Ivern looks like Treebeard.