I would like to use simple collisions to limit the player’s movement, but I’d also like my projectiles to use complex collision to track down where it hits on the map StaticMeshes.
My best idea was to duplicate the map static meshes, make them invisible and force their complex collision to be used as simple collision. But this seems extremely inefficient.
Is there a better way to force complex collisions exclusively on Projectiles, and make the other colliders use simple ones?
I think you are thinking about this wrong.
You have your meshes in your level with both simple and complex collisions. (they can and should have both)
Build out your simple collisions in the meshes to be larger etc,
Then, your character movement / capsule, will use the simple collisions.
And then your projectile hit functions, could be set to only trace/trigger on the complex channel. By default the movement under the projectile component is set to “Trace complex on move” That will achieve what you are after
(these are all defaults btw, so just get those simple collisions setup on your meshes, make sure the “collision complexity” option is set correct on the meshes, likely “project default” and have the project set to both simple and complex (again, default))
Hopefully that should help yah 
probably better to use blocking volumes to limit the player movement.