OK, I want to get this figured out before I do the dragons, so here is what I am thinking. I think we have a basic trait that is then modified by: the amount of damage, the number of conditions, the number of uses. Something like this:
Unstoppable (# of uses). The (monster) can use a reaction, take (#) hit points of damage and immediately end (#) condition(s) or effect(s) it is suffering.
CR Damage/Condition # of conditions # of uses 0-5 5 1 1/Short Rest 6-10 10 1 2/Short Rest 11-15 15 1 3/Short Rest 16-20 20 2 3/Short Rest 21-25 25 2 unlimited 26-30 30 3 unlimited
What do you think?
So I think the 30 damage number felt right at the CR 16ish level where I have used some of your monster ideas, so I would be fine with that.
In terms of number of conditions, again its too much to track (well which effects will I remove, how many do I have left, etc). I think unstoppable is unstoppable, if its activated, all conditions are dropped. Realistically I rarely see that many conditions stacked on a monster at one time, once he's disabled the party moves on to condition another creature....or they just wail on it.
# of uses: My combats very consistently last about 3 rounds (in fact one of my players actually noted that recently himself). Also looking at the boards, 3 rounds seems to be a common duration for your general fights. So again I say...just make it an innate part of the monster, very rarely are you going to eck out more than 3 uses of it.
What you could do is keep the damage high at the lower CRs. So yeah, those monsters can use it as much as they want....except they are REALLY killing themselves to do it, so they are going to be loathe to commit to it that often. So the system balances itself, if the DM wants to keep using it at lower levels to do a ton of damage to his monster, than the party kills it all the quicker.