6-8 encounters every day shifts the game into directions that are problematic to story & setting...
I have given three possible approaches in this thread to dealing with the 6-8 encounters per day recommendation:
1. Chain together encounters so that players use more resources in a single extended combat. A mini-dungeon with four rooms is potentially four encounters, for example.
2. Make the encounters harder or less predictable so that players remain interested.
3. Stop worrying about the 6-8 number and just do what makes sense for the story. This, frankly, is my preferred solution, but I get that it won't work for every table.
... But you don't seem to like any of these ideas, and I'm not sure what it is you want. You say that save-or-die is the only thing that scares your players, but you don't want to use save-or-die.
Help me out, here: what would an ideal adventuring day look like, to you? Two or three combats where the players are scared of losing characters in each one, but not because of save-or-die effects?
Pointing out how someone died in your game to a save or die effect underscores rather than contradicts my point though since I've been saying those effects are the only thing that poke the fear buttons on the player's lizardbrains.
First, why is "poking the fear button" so important? Do you have the sort of players who get bored unless they think their characters might die?
Second, why is save-or-die the only thing that scares them? What about monsters that dish out large amounts of damage, like half a PC's hitpoints per round? Would they work?
Third, my main point was that fights aren't always predictable. There are other things besides disintegration that can affect the flow of a fight; I just picked that because it was an easy and concrete example. I remember a long thread about whether a banshee's wail was OP, for example. Or take dragon breath. If the dragon goes first, that's a very different fight from one where the dragon goes last.
Just like when I mentioned playing games & sports with a toddler, I can do things to make it interesting & adjust the difficulty but I have many more options to functionally do so than MMA reenactments like disintegrate or accidentally/intentionally tripping him to drop hp to 0 then just stomp the body a few times (knock out, first first two death saves with a repeated attack, do it again).
Okay, so if you feel like you have lots of options to make things interesting and adjust the difficulty, then what's the problem? Are we back to "it's no fun if the players aren't scared"?