Mortars have a 30m kill radius and you can hang rounds very quickly. There are guidable versions (expensive) but the regular rounds a cheap as chips (in military terms)
Yes, but you can suppress a trench with mortars and do a combined arms attack against it. As for Tanks, there's other munitions better suited, the only reason the UAF aren't use them is scarcity.
Scarcity is not some kind of minor detail; it's damned near the whole story. Logistics win wars. Scarce weapons usually lose wars against abundant weapons. That doesn't always happen—the Zulus' abundant spears and arrows were no match for the Maxim gun's somewhat less abundant cartridges, and neither a pitchfork nor a handgun will do you much good against a tank—but it does usually happen.
When scarce weapons can defeat abundant weapons, and it's not because of range, it's usually because of precision. Drones are great at precision. 3 mortar rounds falling within 30 meters of your target trench are still ten times less deadly than a single FPV drone you can pilot straight into it.
