I might take a different look at it. Rather than it being a bonus action, have it be a thing that happens at 0 hp.
Unyielding Will: When a PC drops to 0 hp, they can activate this ability once per long rest. They roll and recover hitpoints equal to 3 hitdice (or equal to their max hitdice, whicever is lower), but gain 3 death save failures. The failures last until the next short rest.
The goal is to remove some of the nitty gritty decision making here, and just bring it to a brass tacks moment. Your PC is unconscious, with all of the penalties that brings. OR you can choose to stay up getting a decent check of HP....but you are betting the farm. If you take a death save failure again your dead and gone....so are you taking the gamble?
That way you are removing any of the math and decisions around "hmm 1 hit dice, 2, should I take a bonus action for X or Y"....its a pure cinematic bold decision. Big Chunk of HP....BIG risk of death.
Unyielding Will: When a PC drops to 0 hp, they can activate this ability once per long rest. They roll and recover hitpoints equal to 3 hitdice (or equal to their max hitdice, whicever is lower), but gain 3 death save failures. The failures last until the next short rest.
The goal is to remove some of the nitty gritty decision making here, and just bring it to a brass tacks moment. Your PC is unconscious, with all of the penalties that brings. OR you can choose to stay up getting a decent check of HP....but you are betting the farm. If you take a death save failure again your dead and gone....so are you taking the gamble?
That way you are removing any of the math and decisions around "hmm 1 hit dice, 2, should I take a bonus action for X or Y"....its a pure cinematic bold decision. Big Chunk of HP....BIG risk of death.

