And 1.8k is all the range a CIWS needs.

Well, I’d argue that any air defence system needs as much range as it can get, but for use on a land vehicle for use against aircraft 1.8km effective range is inadequate today.

Most combat aircraft today can’t take anywhere near the smacking that aircraft of WW2 could.

Would disagree there. Some aircraft, like F-16s are made as light as possible, but specialised aircraft like attack helos and ground attack aircraft generally have some form of armour and redundant systems.
Former Soviet aircraft having armour plates scabbed on, like the Mig-27m etc.

And I think the US Navy was smart enough to weigh the pros and cons of going with the .50 caliber (ie it is offered in both a three and six barrel gatling version by GE) or the 20mm.

50 cal projectiles were already failing on Mig-15 type targets in Korea. They went for 20mm for rate of fire and weight of fire. Those gatlings used against aerial targets in the US and by the Soviets are smaller calibre… 20mm in US and 23mm in the Mig-31 in the East. For use against ground targets or both ground and aerial targets both use 30mm guns in the A-10 and the Mig-27 and Su-24. Of course the Mig-27 and the Su-24 use gatlings because they need to fire rapidly because they will be flying very fast and very low. The slower Su-25 has a twin barrel gun with a lower rate of fire and 30mm calibre. The US 30mm gun has more to do with getting multiple projectiles on target as quickly as possible to increase the chances of a kill, despite being fired from a slow moving aircraft.
Apart from podded guns the only other gatlings are on AC-130 gunships in the US or Helos, with the latter being a 3 barrel lightened 20mm gun chosen for weight and commonality… the 20mm round not being overly wonderful for ground attack.

It is not the rate of fire (ie although that is one of them) but the fact that the cannons rate of fire can easily be changed,

No, it is rate of fire. Although it has an added benefit that rate of fire can be changed, in practise it is generally fired at one of two rates so the true benefit of controlable rate of fire is never realised.
An A-10 will have a short space of time hanging in the air over the target area to fire a burst of 30mm cannon shells into the top of the target. The more shells it can fire in that burst the better the chance for a good kill.

It just keeps firing but not that barrel. And the USN and USAF thought of SABOT rounds for the Phalanx and the A-10s cannon but decided that a better option was a standard AP with a core surrounded by an aluminum body.

The A-10 is a rather simple unsophisticated aircraft. Firing sabot AP rounds and HE rounds with a completely different trajectory would make firing runs less flexible as half the rounds in the belt would go high and half would go low… and sabots in the engines could ruin the pilots day.