I kinda would love to sit in on one of your games to see how exactly you get to do that. It is so different to my experience.
What party size are we talking here?
The account below fills me with questions.
They certainly get frustrated by TPKs, just like they do when a combat is over before they got a chance to act.
If I were going to break it down, I probably have the following (across all groups) with 5e.
Fight that ends before all characters get a chance (10%)
1)
What do you mean by this?
My interpetation would be that this is a fight where the bad guys (or the characters) die in the first round before all characters act.
Fight that ends within two rounds and you don't even have to care what's happening (70%)
2)
My take on this is a fight where most of the enemy is killed in the first round.
Is the party nova-ing?
Fight that feels like a "good challenge," party has to use some strategy and resources (10%)
4)
It is a good challenge for whom? (you or the Party)?
What is strategy in this context?
What level of resource use?
Fight that is custom built by me, getting lucky, throwing out the system's guidance entirely, that ends up epic and fun (4%)
5)
Why should this number be bigger?
6)
Are these failed attempts at epic and fun?
To be honest #5 - epic fights should be uncommon, it is narratively implausible otherwise and mathematically unlikely. If you try for it will result in excessive TPKs.
#4 should be your default, at least for 5e. Even for grittier game systems if you murder the party too often it will not be satisfying.
Gritty campaigns work best where the party choses combat as a last resort and try to only fight on their terms. If your players want heroic combat and you want epic combat the maths is going to be against you in most systems unless the system allows the turning of defeats into something else (or you do it as referee).