Avoiding Death Spirals WHILE making damage count

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A very simple explanation of the problem:
Right now it feels like to make damage feel like it counts, then you would suffer effects other than HP loss when you take damage. The argument of "funny how you have 100% your fighting capacity even at 1 hp, but then 1 tiny wound later and you're unconscious." Right now it feels like monsters and many PCs are just bags of hit points where they don't really mean anything until that last shot.
But on the other hand, if you start doing things like wounds, or detrimental effects, you start a death spiral, and based on feedback I've received, no one seems to like those.

So what's the answer to that?
How you make hit points, and hit point loss matter beyond that binary >0 or =0 determination while also avoiding death spirals?

A week or so ago I mentioned how you could have two layers of "health". You've got something like vigor, which is your combat staying power. Then you've got your actual health, which is measured in much lower values. When your vigor drops to zero or less, you reduce your health by 1 and take a fatigue level, then roll 1 dice for every two tiers you are based on your vigor die, setting that as your new current vigor. Drop to zero health and you die. Vigor recovers fairly quickly on rests, health much more slowly.

Example: Bob the warrior is an adventurer (tier 4, d8 for vigor die). Bob has 22 vigor and 3 health. In combat he takes 16 damage. Vigor is now 6. He takes an additional 8 damage (more than his current vigor). His health drops by 1, he takes one level of fatigue, and rolls 2d8 (1 dice for every 2 tier levels), making that his new current vigor. (for you math nerds, this means a Bob would have a total of 8d8 vigor before health goes to zero)

Every level of fatigue reduced your movement and imposed a -1 penalty to all attack/skill rolls.

But that right there is the death spiral. And there's not enough...meat there to warrant separating vigor and health.

Q: Why not just keep hit points if that's the case?
A: Ok, to address this, you can use vigor (since it's kind of like endurance and adrenaline and willpower all into one) to fuel some of your PC abilities. Want a bonus to attacks? Or increase movement? Or overpower a spell? You can use vigor for that.

Q: OK, but what about that death spiral?
A: Now we get into the real question. We know taking a penalty every time you lose health is part of a death spiral. There are two solutions I can see for this:

1. Some classes have traits to avoid taking fatigue. E.g., a fighter can use a defensive stance that prevents fatigue from being imparted when health is reduced if they want. Others can choose a class feature if they want that helps. The key is that players have the choice to go more conservative and avoid fatigue, or go more aggressive and use their choices for more offensive tactics, risking fatigue as the drawback.
2. The second is something I was working on when I was writing Bugbears&Borderlands OPEN: Desperation points. Every time you lose health, you also gain a desperation point. If you current health is half or below your maximum total, you can use a free action to spend a desperation point to gain a benefit. Choices include but are not limited to damage reduction, extra movement, skill bonuses, remove a negative condition, bonus spell points, etc.

Thoughts? Does this achieve a goal of making hit points actually count and taking damage have a factor, while also mitigating the death spiral?

Another approach could be increasing lethality - i.e. the risk of continuing to the combat - rather than imposing a penalty (or bonus). With that in mind I plan to playtest something like this

1. Below half your hit points you have the wounded condition.
2. Some weapons and other effects (e.g. spells or monster attacks) care about that condition.
3. The anchoring definition for "care" is to roll one more of the base damage dice (e.g. roll one more d8 for a longsword)
4. With the option to escalate that on about the order of magnitude of
   1. additionally inflicting one death save failure if the attack causes the target to fall to 0HP or lower OR
   2. force a roll on the Lingering Injuries tables OR
   3. inflict a different condition like prone

So my idea here is that the wounded condition itself does nothing. Instead, it provides a handle for a group to connect other rules to, for example weapons with additional lethality. This is based on the PHB 197 call out that

When your current hit point total is half or more of your hit point maximum, you typically show no signs of injury. When you drop below half your hit point maximum, you show signs of wear, such as cuts and brusies. An attack that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious.

Elsewhere it is stated that

hit points represent a combination of physical and mental durability, the will to live, and luck.

And - regarding NPC and monster HP - it is suggested in the DMG that below half hit points a creature be described as "bloodied".

How does my suggestion meet your goals?

• There is no death spiral in the sense of accumulating penalties making success less likely and leading to a depressing downward trend.
• Instead, combat becomes more lethal once you burn your will to live and luck, and are down to your physical and mental toughness. The end in such circumstances may come suddenly.

The intent is that players will prefer to find a way out of combat once on low hit points. I have contemplated also tweaking some healing spells - such as healing word - so that the can't benefit creatures with the wounded condition. Whether that is a good idea or not, I think it shows how simply creating a signifier in the system to represent the way hit points are described, and then giving that signifier no properties, but rather letting other system features trigger additional behaviours based on it, might afford strong designability which can be cashed out by groups in different ways.

The way I want to cash it out is to have characters reach a point where they are "bloodied" (be that physical or spiritual) and start to see that although they can continue fighting (without having to track sundry bonuses or penalties) doing so will come with escalated risk against some foes.