the main issue i've had with performance in recent playthrough was that eventually my defense weapons upgrade the range to the point that the enemies are dying outside the range of the Pickup Tower I.

I can upgrade to next tier but eventually CPU becomes a problem.

one idea was to change the tower so it scans only a rectangular area in front of the tower - so it could be rotated behind a wall and have area of effect be OUTSIDE the wall.

this might help with scan efficiency since it will no longer be scanning empty ground/entities inside my defense perimeter.

Current pickup tower is comprised of two entities: container (chest) and radar. But none of them support rotation or direction.
It is not possible to assign direction, without using a third entity or extra variables to the global domain.
I think neither of them are good solutions to this.
(Adding third entity makes the build/destroy script cumbersome, and has the problem in selection, where you are selecting the chest part to access inventory or the rotated part to select direction?)
(If adding external variables ti the global domain to store the directions, they have to be handled on mod load/changes. That would be troublesome, so I try to avoid "store external variables" unless absolute necessary.)

PS: Many things are not rotatable or has direction in Factorio. Just like I have thought of applying directional light, which would be useful along side with turrets to illuminate kill zone. But game API does not allow assigning direction to my modded lamp. So I have to give up the mod idea.
If the future game API allows that, maybe I will revise these.

the other idea i had was a combat tower to defend itself, then it can be placed freely outside the wall.

Hmm... Again, this would be a compound entity of three entities: container, radar, and turret.
It is not possible to avoid the first two: container for putting in the loots, radar for the periodic event trigger to pick up.
The turret will compete with container on the selection priority. Either the player can't see the storage and put wires to it, or the player can't see the turret condition.

If it is an ammo-consuming turret, it would have the additional problem of separating loot from ammo handling.
Some mods use scripts to handle ammo stacks, but that adds burden to UPS. Because there are no events that are triggered by container content changes nor inserter actions. (For example, just a few creative chests can consume already quite a lot of consuming power, even if no actual content changes are needed.)