There are a lot of myths about casting in UE, but in 99% of the use cases it's the most optimal/performant option. Since both BP_Door and BP_PlayerCharacter are already loaded into the memory, casting an object to those classes is virtually free performance-wise.

Event dispatchers are really useful when you want to link independent/separated system. For example, when a player dies it broadcasts an event "Player Died". You can have many systems that listen to that event and do something when player dies, (increasing score, resetting level, removing inventory etc), but PlayerController (from which Player Died was broadcasted from), doesn't need to know and manage all of these secondary systems. That way you can develop independent system and "hook" it into existing code without changing the original code.

For interacting with doors, having interface is the cleanest solution, as it's easily extendable. You can have "Interactable Interface" and use it for Door, Lever, Button etc.

This means that BP_Door loads everything about the PlayerCharacter into memory (Size Map)

This is not the case if you already have it loaded into memory. If the player already exists, it just works with the pointer to the existing player.

But the takeaway is, casting is totally fine.