Hawk Diesel
Adventurer
You're way over thinking it.
Just make all short rest abilities into either 2 or 3 uses per long rest & let the players decide how quickly they exhaust their resources.
And your idea concerning 6th lv+ spells is just bad.
Consider:
* It's game night & my players are trekking through the jungles of Chult on foot. In 4.5 hrs of play they cover about 7 hexes worth of travel in the session. (roughly 10 miles/day) They have 7 encounters overall.
And in a weeks worth of adventuring our wizard player is allowed to cast her 6th lv spell once in all that time - for no better reason than "No matter how much time passed in-story, it's still Monday 2/12/18 IRL".
*
* It's now 1 session later & the party has found a dwarven mine. They end up spending this + 3-4 more sessions clearing it etc, representing only about 8 hours of actual elapsed game world time. And yet in those 8 hrs our wizard has been able to cast that 6th lv spell 4, maybe 5 times?
Blech.
You're absolutely right. In games where you actually follow the structure of 6-8 encounters per adventuring day, there is no need to adjust the system. What I am proposing is for those of us who find that our gaming experience differs from that standard. Since the game is balanced around that idea, the balance flies out the window if your game structure happens to differ.