I personally prefer top level subvolumes (@, @home, @var, @var), because it makes it easier to know which system folders are subvolumes and back them up accordingly. They are then mounted at their respective location under /.
E.g… I do snapshots looking at the btrfs filesystem and its top level subvolumes. I’m not doing snapshots going from the mounted root filesystem. I.e. I’d do a snapshot of @home, not a snapshot of /root/home.
If you want to use backup/snapshot automation tooling, I’d recommend looking at how they expect the subvolumes to be set up. E.g. snapper and timeshift expect a specific layout (which can stil be done manually after OS installation, but why bother).