Hi @wkrt!
Welcome to the community! Feel free to Introduce yourself 🙆 to the community if you want to! 
I’m not an expert on Unreal/Unity, so maybe @Jedd can jump in on that one 
To use Reality Kit, you are correct, you’d have to deal with all the calls through the API. As far as I’m aware there hasn’t been any efforts on iOS integration as of yet (but we’d welcome a community connector for iOS 
)
Dealing with geometry can be a pain for more complex geometry types like curves, surfaces or breps. But for all those types we provide fallback meshes for programs that don’t support them.
I’m assuming all AR stuff is actually done with meshes/polylines, so you could skip most of the complexity by just using the fallback values whenever you receive anything that is not a mesh or a polyline (this is what our online viewer does)
If you’d just want Receiving capabilities in your app, it should be fairly straightforward to implement
You can have a look at our Dev docs (Introduction | Speckle Docs) and feel free to reach out for any questions you may have 