Hi @evelyngrace1!
It seems that you’ve already narrowed down the issue- your objects are too high-poly, your VFX systems and dynamic lighting are too complex, and your animated assets are hurting performance.
The solutions here are to reduce poly-count, reduce particle count and shader complexity on your VFX systems, disable dynamic shadows on as many objects and movable/stationary lights as you can, and check your animation blueprints for too many references or any costly on-tick functions.
There are many settings that can be adjusted per-platform or per-scalability level to help you gain performance on lower-end iPhones without sacrificing quality on higher-end devices. Here are some links to investigate some possible methods:
https://dev.epicgames.com/documentation/en-us/unreal-engine/setting-up-per-platform-lods?application_version=5.1
https://dev.epicgames.com/documentation/en-us/unreal-engine/optimizing-lod-screen-size-per-platform-in-unreal-engine?application_version=5.1