I feel like this is just really doubling down on the key problem here, which is that D&D has saves that are at once, way too specific, and not specific enough to be used in the way you want to use them.
In what way?
So with your stitching logic, which is not entirely worthless, I agree, you'd what, just instantly kill the PC?
No, that is the point of saves and assigning the DC. If a poison is so potent as to instantly kill, either the DC and/or the damage would need to be high enough to have that be the case
most of the time.
Or would you make a save? But if it was a save, why on earth would it be a CON save? Basically the only thing that can save you is dumb luck.
Sure, again that is the point of saves.
You could go many routes IMO depending on how the DM wants the nature of the poison to work.
1. The PC makes a Wisdom (Perception) check to notice the
almost imperceptible poison. The DC should be sufficiently high, at least 20.
2. Any other PCs around the chest could also make a check as well, perhaps warning the first PC
before the chest is touched.
3. Perhaps a DEX save for the PC to pull back their hand before touching it?
4. Assuming you actually touched the chest at this point, it is a CON save because it
can be resisted. As I said in my prior post, CON is just one part of it, proficiency (i.e. skill, experience, etc.) is also a part, but unless the DC is low enough and the bonuses high enough, luck is the biggest factor.
For example, a couple weeks ago I was driving in bad weather (snow, low visibility, icy roads) but the snow plows were out, sanding the roads, etc. so the conditions weren't as bad as they otherwise would be. The two factors effectively cancelled out the disadvantage (bad roads) and advantage (plowing/sanding), so overall any Dexterity (Drive) check I make would be normal.
Now, other people were driving more slowly, and at one point I moved to pass another vehicle. As I rounded the curve of the highway, suddenly there was a truck along the roadside, half in my lane! I had made my check, driving safely, but with this I had to react, and so it would be a DEX save. My reaction speed is good, above average, so because of that in "making my save" it plays a part, but luck is also there.
I was lucky I wasn't looking down at my speed, lucky I wasn't changing my music, lucky the car I was passing slowed down enough to allow me to avoid the truck by partially going into their lane. Any one of these factors (and many many more) could have contributed to my failing the DEX save.
In the poison example, there are many factors still: how much poison is needed to kill instantly, is it
always lethal (high DC, so possible to not be killed) or can a good constitution give you a chance (however small) of surviving it, is it possible to build up a resistance, etc.
Many creatures' biology makes them more or less affected, so how does that play into it?
So, a saving throw is partially skill, ability, and luck each. If you have a high enough skill and ability, you can always save because the DC is low enough that even a 1 makes it. Likewise, it can be high enough that regardless of skill and ability, even a 20 fails. So, luck isn't always a factor, but only those times when success or failure is automatic.