Maybe G5 is the mastered haki form or its the awakening which doesnt utilize haki
by this i mean, what if there is nothing to master about G4? Like what if G4 just works that way no matter what. Like the actual mechanics of how it works just plainly lead to quick exhaustion and there is nothing to actually master here
So you just need a whole seperate ability instead. G5 could just be the ability whose mechanics are entirely so different from G4 that they just lead to better haki use and its G5 thay doesnt have a limit.
You’re question then is, “why would Luffy ever use G4 then if he has a wholw separate form that doesnt have a time limit?” Well then maybe its not a full form the same way G4 is so he cant just do extended combat with it
Like remember how G2 and G3 were introduced together, where G2 was a full form that just comes with extended combat potential while G3 was sort of a form but it was mostly augmentations of Base luffy for specific attacks.
What if G5 is like G3. The reason Luffy doesnt just stop using G4 is because G5 isnt an Overal stat boost like G4 and so its not actually useful for full on extended combat. But then since its not used for full on extended combat, it is also not using haki nonstop like G4. So basically G5 could be base luffy doing something very interesting just in base form and augmenting some few stats like G3. And if G5 also has a drawback like G3 used to have, its not a haki bases draw back. So Luffy would essentially mever run out of haki using G5 but hes also not getting that full stat boost for extendes combat of G4.
Something like that
Or it could be just awakening
oden s haki 
using it against a ship and a number 
