Is your problem that fights don't feel very dangerous when heroes have lots of hit points? Reduce the maximum number of hit points or increase monster damage.
Pretty sure this is a no go as already stated by Xoth. 5e players looking for a 5e game expect some certainties in their 5e games. Some aspects are crucial to the perception of playing 5e, I think Hp progression is one of them, even if many agree that it’s an abstraction that can get in the way of certain story telling narratives.
Many previous additions of D&D going back to AD&D were aware of the limitation of the HP abstraction, and how it could take the drama out of certain scenarios .Look at the AD&D assassins ability - that is actually intended to be applicable to all classes in conditional circumstances, such as attacking a sleeping target.
Doing big damage is the other method, but again that changes the traditional expectations of 5e players too much IMO, and the design of some classes extra damage would be lost in the multiplication.
What’s required is something that can slot into the existing 5e framework, without having to alter the rest of the game. Which is what’s been arrived at. Standard 5e models high heroic fantasy, it’s not designed to model S&S which has a darker bloodier edge to it.
I think we are still very far away from the more simulationist approach of RuneQuest, or the very grim and gritty wfrp. This is clearly still DnD in its cinematic style, it’s far from Rq and wfrp.
I must reiterate you appear to underestimate the signal you're sending. By including a variant rule in your document you're sending the signal "this variant comes so highly recommended I bothered to include it in my document."
But it doesn't appear that is your intent. I would argue the best thing is to not include any optional rules in a document like yours. Either you stand by it "this rule is essential for my product" in which case it should be presented as core to your setting, or you don't and you simply leave it out.
Sure there is a market for optional D&D rules variants, but this isn't that kind of product, in my humble opinion. I suggest a laser focus on what is essential to the Xoth experience
Well there are different opinions and how to create that S&S atmosphere.
If you want a high heroic game where there is little danger after 6th level then use 5e as is, that’s its standard intended mode.
If you wish to alter that expectation slightly and add in some more S&S thrill, danger, and excitement, then I think Xoths suggestion does that elegantly without redesigning 5e. The death saves are already a safety net for player characters in 5e. Changing damage expectations, or hp progression is going to be too big a structural change for the majority of 5e players. Take a look at other 5e designers - AIME, Trudvang, symbaroum, Beowulf. The changes they make are slight but impactful, the basis of 5e remains the same. 5 torches deep is an example of a game where the structure of 5e has been overhauled, low hitpoints more in line with b/x, but it becomes a different game, it’s no longer 5e.
Besides it’s already been pointed out as being an optional rule. Really can’t see the problem here. Optional means optional does it not? That’s the the benefit of working off of a strong but malleable 5e frame work. DMG is full of such suggestions, but they are all optional.