When it comes to the level 5s, I think Clark mentioned it once after the battle of diamonds, when he recruited the mc to help kill a monster in the sewers, about how the fight was breeze, but the travel was a pain. I think aside from the big 3, the other lvl 5s aren't fast enough to respond to an attack in another sector before damage occurs, so they have to rely on the future predictions, and wait for the monster to appear, or play hide and seek with it until they find, it which eats up their time.
Also, remember what Stephen said that it's not a few thousand but a few handfuls that are protecting the planet, if every monster attack in every sector has more than one A rank, that implies that they are deploying the lieutenants, and captains, in addition to the clone units, to every attack, and without Nico constantly acting as a taxi for the others, they will always be slower than the teleported monsters leading to casualties.
In conclusion, I think the reason the level 5s are kept busy isn't because it's difficult for them to kill the monsters, but because they are always playing catch up on a planetary scale. I mean imagine if you are playing a game of whack a mole, with an unknown time limit, and every time you fail to kill the mole the first time it appears, you lose points (which in this case is lives or portions of the planet), wouldn't that drive you crazy? When you don't the time limit won't it feel like a game you are already losing?