We have, in the "endless" WZ lvl, 2 major problems, whereas the 2nd much depends on the first, but not entirely.

1.) The time limit
2.) Difference in levels "rolled" (size difference)

1.) Time time limit:

I see you wanted to make it challenging, and in a way that allows players to fail, but the way you did this is horrible.
Think of running lvl.... 120 or so mission and it would have taken 15:02 to kill all mobs and you fail,
you think that is motivating or rewarding?!
I did not think so.

A little excursion on gaming history:
Look, you missed a lesson here that was already known to Bli$$ard when they released WoW back in 2003 or so.
When they implemented the game, they did not want all the kids to starve in front of their game, or sit there 12h a day and drop out of school, not because they care for the kids, but it would be bad publicity, so they had to input something that makes it at least not look like they want people to grind WoW 24/7, and they did a "rested xp bonus", which was originally designed to be an experience penalty for playing too long, but guess how that would have been received by the players!
So they did in fact an experience penalty but called it a bonus instead, it does the exact same thing but has a positive wording to it.
See the difference?

So instead of "fail if not done within 15 minutes" you should have made it a bonus for doing it under 15 minutes,
you could go from there and give even bigger or more bonuses for doing it under 14 minutes, under 13 minutes and so on...
And players would have to figure out what bonus and time per mission is good or better for their actual goal.
You could put in even more diversity by making the bonus more favour oriented, so aiming for that would max favour,
and aiming for higher lvl missions would max xp/time (would need a different xp scaling than what we have currently also).

Then you could have put in meaningful perks and abilities and not the trash we have there currently, and force players to decide if dumping a lot of favour to buy "more damage mark VI" for xxx favour for this mission to do it in xx minutes would be better or worse than doing a lower level mission with less expensive perks/abilities.

Well, I hope you get the point.

2.) Difference in levels "rolled" (size difference)

You may have noticed that the levels you get are not all exactly the same, and the time needed to complete them may vary by 100% more time needed for the bigger ones comapared to the smallest.

This is not only completely retarded in relation to point (1.), but also in relation to maxing out reward per time spent,
if players can save 5 minutes by re-rolling the mission 2-3 times, then they will do it.

This not-thought-through design forces players to accept and surrender missions more than actually playing them.

How could you have missed that one?! I really can't even imagine.

Don't you test things even a tiny bit before you release them?!