What happens if one node on your network is downloading at 1Gbit and another is uploading at 1Gbit?

Both get 500Mbit.

Bottleneck.

That's going to be super rare. If it's just LAN traffic it shouldn't hit your router at all and you won't have the bottleneck issue. The actual cases would need to be quite contrived, like you're backing up your media library at the same time you're updating cod warzone.