the low-hp scenario is why I included the bit about sacrificing death-throws for added damage. the idea is that if you sacrifice all 3 death throws, you’d have reached your massive-damage threshold anyways. WRT the AOE, its pretty big. assuming you don’t have any levels of exhaustion, and you want to survive, you get a 30 foot radius (5+5*5).
so say your character has a max HP of 90, but you’re down to 10, and you’re not exhausted (which is pretty common, i think). you could deal 70 damage to all creatures in a 30 foot radius, and still survive (unconscious, severely debuffed, and with 1 remaining death save). or 100 damage to all creatures in a 35 foot radius, and die.
but the fact that it wasn’t clear to you is a pretty good indication that this is probably too mechanically complicated to be actually fun :(
also you raise a good point about Death Ward. Thematically it would be fine - you’re “willingly sacrificing” your health, rather than taking damage, so apply it directly to your hitpoints rather than temp hitpoints, bypass all those kinds of defenses, etc… but that just adds even more mechanical complexity.