Another option is to make it a little less… stackable with other things, and potentially growing more powerful as you level.
When you reach or go below half of your maximum HP
(maybe mention before Lethal Damage, or add in "current maximum HP" to make it clear how Lethal Damage would interact) you gain the following based on your highest Attribute Score:
When you reach or go below a quarter of your maximum HP, the Advantage amount increases by 1.
In addition, this counts as Bolster, and thus does not stack, you may choose which effect stays.
Or keep it all the same, but take away the quarter thing, and just increase each one by Advantage 1. Since it no longer would stack with Bolster, it wouldn’t be as powerful, so more advantage might make sense?
OR, maybe with it scaling based on your Score, just taking the quarter thing away altogether.
Just a few thoughts