I confirm you observation when trying to reproduce it.
You would need to select the third option, the blivet editor.
Here you can select the btrfs volume in the left hand culumn, and then you can delete the root subvolume and recreate a new root subvolume. For the home subvolume you just need to define the mount point, For the non-btrfs partitions you need to define a mount point, and perhaps also chowse format.
When chosing “Manual partitioning” don’t lete you format the root files systemd, and it also don’t allow you just to remove the root subvolume (unless there is a secret way to do it).
This may (hopefully) change in some future version.