No, no, no... you completely miss the point. I really didn't think I had to take this aaaall the way from the beginning, but here goes.
If your goal is to have 6-8 encounters before the long rest, it doesn't matter whether these happen over an actual day in a dungeon, or over a month during a gruelling desert trek.
They require different pacing. If a story plays out over years, you can't play through 6-8 encounters every single day, everyone will be 20th level (or dead) long before the climax. If you're running the fantasy version of 24, you might have 30 encounters in a single 'day.' You might sack out every night in the years-long story, or not slip a wink in the 24 story, so you're not changing rest frequency (much), it's the pacing of how much /action/ there is between any two rests that varies.
OK. I get what you're attempting to do now. Although I still have issues with the solution. Why? Because you still haven't provided any meaningful fiction to explain why I can't stop and take a rest for a week. Because at night in the desert, I'd pitch my tent (or Leomund's Tiny Hut), and, well, rest. People who live in the desert rest. People traveling in the desert rest.
Sure, you can say the Desert of Thirst has no water, and that sounds like an effective method to cause difficulty, especially since you can't take a rest if you don't have sufficient food or water. Except if you have a cleric or druid, in which case you have water, and you can rest.
So let's remove the first problem I've got with it. Let's separate it from a rest - resting (a normal thing that people do on a regular basis), let's tie recovery to time. You can use the ability again every 4 hours. Except that doesn't solve your problem.
So instead, you're proposing that on a day when you have more encounters, you recover faster. You are able to recover after 5 encounters, let's say. OK, on a normal day that's fine. But when you're traveling, you can't regain your spells now for days, maybe even weeks or months if you don't have any encounters.
So let's try to look at it from a real (fictional) world. Spell slots require you to reconnect to the magical energy, which takes either time, rest, or sleep. I use time and sleep in my campaign, you regain spell slots while sleeping, or after 24 hours if you don't sleep. This is part of the "physics" of the world and I expect that to remain consistent whether I'm gardening, shopping, traveling or adventuring.
AD&D addressed this with much, much slower healing (pretty much requiring magical healing), but otherwise is quite similar (other than having fewer recharging abilities).
1e Recharging Abilities
Spells (see below)
Clerics could turn undead at will, but only once against any given undead.
Druids at 7th level could change into one of three forms, each once per day, although no time restraint.
Paladin lay on hands once/day
Cure disease once/week per 5 levels
Recovery of spells was slower. You had to have complete rest (usually sleep), for up to a certain amount based on the highest level spell to recover: 4 hours for up to second level spells, and 12 hours for 9th level spells, to go with the 8 hour long rest, you could recover 6th level spells.
Then it took 15 minutes per level of spell for each spell to memorize. So a fifth level wizard (4, 3, 2) would need to spend 4 hours memorizing spells. The same for the cleric. In 1e it was 4, 2, 1, so it would have been 2 hours and 45 minutes. So that's either 10 hours (5e spell count) or 8 hours and 45 minutes (in 1e).
Now in my campaigns, we assume about 8 hours of adventuring daily. So setting up camp for the night, resting, memorizing always fit in that approach unless you were very high level, and needed to memorize a lot of spells. So that really hasn't changed anything in my campaign. Even though you don't need as much time to prepare spells, we still rest the same amount of time.
You regained no - zero, zip, nil - hit points resting. You had to spend a full day resting.
Now, let's look back at 5e. Short rest abilities are expected to be available at least every other encounter. I believe the game is designed to support every encounter. Is it really that much different? The only difference I really see is how quickly you heal hit points.
So I'll make the same suggestion again:
Use the gritty realism approach for healing, and the regular resting approach for regaining spells and abilities. Guess what? That's almost identical to AD&D.
So beyond that, what other options do we have? I'll go with my default one - the exhaustion track.
Traveling is tiring. The environment (such as a desert) can have an impact too. Guess what, that's what the DMG uses too. Exhaustion is a perfect solution for a number of reasons. First, it impacts the effectiveness of the character's abilities. Instead of saying "you can't" do this, it just makes it harder. Second, RAW anyway, you can only recover one level per long rest. Perhaps you need lower thresholds for them to come into effect in difficult environments. Third, even a 3rd level spell only restores only 1 level.
Where does that leave us? Actually, it's
better than AD&D. In addition to the slow healing, now we're adding exhaustion to the equation.
AD&D didn't have anything that would address the fact that you had fewer encounters per day on a long trek. It did make such encounters seem trivial. My solution was that those encounters were much more difficult. Since there was only one in a given day (or couple of days) they could be really tough. Now, while this sounds like I'm being punitive, or designing it to take into account their power level, the reality is that my reasoning has nothing to do with that.
The wilderness is just dangerous. It's the home of wyverns and dragons, and other great monsters. Caravans travel in large groups, with a dozen or more guards, maybe twenty. So if you're a band of bandits or brigands, you number 30 or 40, not 4 to 6. Which means traveling as a party of 4 to 6 is very, very stupid.
I didn't come up with this on my own. That's what AD&D recommended. Bugbear No. Appearing - 3d12; ghoul 2d12; gnoll 2d10x10 (yes, 20-200 the same as a bandit/brigand).
Sure adventures were designed with the levels of the characters in mind. But low level adventures were close to civilization, and high level adventures further away.
The Lost Caverns of Tsojcanth says it's designed for 6-8 characters of 6th to 8th level. The pregen party is:
6th level dwarven fighter
7th level human cleric
8th level human fighter
9th level halfling thief
4/9 elven Fighter/Magic-user
4/4/5 half-elven fighter/magic-user/thief
The wilderness encounters include (these are all individual encounters):
3 bears
32 gnolls (raiding band)
6 giant eagles
20 Mountain tribesmen
1 Giant snake
70 Goblins
3 Hill giants
12 wolves with 2 wolfweres (basically a wolf that can assume human form instead of the reverse)
102 Mountain dwarves
1 Hermit
4 trolls
2 wyverns
81 hobgoblins (a "small group")
etc.
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Your Solution
Use the RAW for rests and recovering abilities
Use the Gritty Realism rest variation for hit points
Use the rules for environments, and you can create similar effects (a cursed tomb imposes exhaustion levels over time, for example).
Make your wilderness encounters more deadly.
And I'll be honest, I didn't expect these results either, and I may be simplifying a few things in my rules!
