The rune just straight up gives to much flat damage. I don't actually think the true damage part of the rune is what makes it so powerful, instead I think it's the free AD that the rune gives due to how it interacts with bruisers ability scalings. Since bruisers naturally have high AD scalings (due to them usually building mix of tank/attack speed/ad), giving them 60 AD outright for free is always going to be to powerful.
The simple solution:
* Remove the bonus AD that the rune gives.
20% true damage is more than adequate to deal with tanks (which is what the rune is intended for). The bonus AD is overkill and unnecessary. If you remove the AD scaling and leave it at 20% true damage then we'll have a really solid rune for bruisers to run.