When DMing 5e it took me a few levels to find out I need to bring mostly "above-deadly"
opponents to make it any challenging.
The reason you need to use deadlier encounters is because youre having less of them. Long rest resources (rage, smite, spell slots) can be spammed every round, meaning you need to increase the difficulty of your encounters. This also devalues short rest resources even more.
1) the game rules do not force the 6-8 encounter day in any way form or shape. There's nothing even close to formal aids to help the DM out. She is completely on her own in creating story based timers.
The Dungeon Masters Guide. Its literally in there. Whole chapters on designing encounters, the 6-8 encounter/ 2 short rest expectation, timing them, policing the adventuring day. Its all there. Pick it up and give it a read.
There is also rest variants suggested as well.
2) this is utterly incompatible with wilderness exploration adventures ("hexcrawls") or long distance travel adventures
At NO stage is it suggested that you are supposed to pigeon hole the players into 6-8 encounters every. single. adventuring. day.
The general default is 6-8 with 2 short rests. Around 50 percent of the time (any cavern/ dungeon crawl, forest trail, module type affair). The other 50 percent of the time its single encounter adventuring days, 1-3 encounter days, or even the occasional longer adventuring day.
No-one is talking about repetively ramming 6-8 encounters into every adventuring day mate. Its setting a 6-8 encounter/ 2 short rest default
so your players police resource mangaement accordingly.
If you routinely throw single encounter days at your players they'll react accordingly and nova the crap out of your encounters. Problems will ensue with class balance and encounter difficulties.
If you routinely (around half the time) throw 6-8 encounter days with 2 short rests at them, they'll police themselves accordingly -
even on the shorter days.
no official adventure follows this guideline AT ALL
Thats not true. Literally every dungeon area follows this guideline. Bigger picture wilderness stuff might only get you one per day (if that).
Not doing so amounts to sticking your head in the sand; stubbornly parroting the DMG calculations as if just repeating it enough times would make it happen.
What? Im citing the DMG on encounter building (which youre suggesting above doesnt exist). Now your problem is that Its being said too much?
Do you want me to do a mathmatical breakdown of how the classes balance best at the 6-8 encounter/ 2 short rest mark?
I'll happily compare a Warlock 10 with a Wizard 10 and a BM Fighter 10 with a Paladin 10 over 6 encounters of around 5 rounds each with short rests every 2 encounters.
Is a long string of individually trivial encounters (where the only challenge is to conserve resources) any fun?
What are you talking about? Having more encounters between long rests takes no more time than having those exact same encounters with multiple long rests. Im not saying you need to cram more encounters into a play session. Im saying you need to get more encounters between long rests.
Theyre totally different things. Youre not having more combats at your table.
Youre just allowing less long rests.
And Im sorry to be the one to tell you, but DnD is a resource management game mate. Always has been, always will be. HP, GP, XP, Spell slots, charges, powers, manouvers, HD, daily abilities etc etc.
Whats different this time to the last edition is that the classes use different resource recovery systems. Some classes benefit greatly from short rests (regaining all improtant class features), some gain virtually nothing from them. Some are almost entirely long rest dependent classes.
A single encounter day favors a Wizard over a Warlock to the nth degree. A day featuring 15 encounters with a short rest after every encounter favors the Warlock. Surely you can see this?
The issue is finding the balancing point. Its at the 6-8 encounter/ 2 short rest point.
Now no-one is saying that you have to force that encounter pacing on your players all the time. But you need to do it enough so that they pace themselves expecting this paradigm to be true. You need to have THEM police the adventuring day and resources, not force the 6-8 encounter day on them all the time.
Sorry, but the level of advice which basically says "do 6-8 encounters and all problems go away" is incredibly and frustratingly short on discussing the overall picture.
What is this 'overall picture' you want to discuss?