The resource recovery model / resting rules are my biggest single problem with a game I otherwise love. Actual play holds up the 6-8 with two short rests that the DMG recommends as a good balance point between the at-will classes, the short-rest classes, and the long-rest classes. But really, regularly running that many in a single day often does not fit the narrative I want to tell or the pacing I wish.
Apropos of very little, there actually aren't a lot of short-rest classes. Warlock, Monk, BM Fighter. At will? Rogue (other than AT), Champion Fighter.
Ask yourself, can we live without the above? Just EKs, ATs, and the other remaining full classes?
I think the answer's 'yes.'
At that point, you just have to worry about the balance among the full casters, 1/2 or 1/3rd casters, and the n/day Rage-bomb barbarian.
There's how many 1/2 casters? Yeah, majority rules. Just run all-full casters. Now the only deal with pacing is how hard the encounters are.
D&D. Fixed.
But, who'll do melee? War Cleric, Valor Bard, Moon Druid, henchlings, hiremen, golems, pets, companions, summons - whatever the Artificer or Necromancer cooked up this week.
Doing fewer occasionally is fine - as long as you do more just as often. Alternating <6, 6-8, and 9+ encounters between long rests will let different types of characters shine on different days.
Yeah, maintain that average can be taxing. A single-encounter day requires half a dozen 9 encounter days to offset.
There is a persistent myth that fewer, more difficult encounters keep the balance between the classes. This is because it can keep the same level of deadliness and does force attrition.
Barbarians, for instance, do pretty well in that model.
But really, I'm spoiled by how 13th Age does it.
13A is generally good design. It set out with many of the same goals and parameters as 5e, but generally found better solutions. Full heal-ups are just one example. Handling TotM's another big one.