Yeah, kind of hard to figure out what’s going on without being able to look at your project.
The only suggestions I can think of is to either use World Partition HLOD to switch to between the instances with collision enabled and the ones with collision disabled, (with the downside that keeping the instances in the HLOD synced to the non-HLOD instances is difficult, but not impossible), or start with collision disabled and get all Foliage actors within a given radius of the camera and enable collision on them after the world has finished loading.