Looks like UsrMove fixed the problem with separate /usr volume.
This is the layout I tested:
UUID=5AC5-5766 /boot/efi vfat
x-systemd.automount,noauto,umask=0077,shortname=winnt 0 0
UUID=8fd67b94-0dd5-4df0-a690-066460e47fd8 /
btrfs subvol=root 0 0
UUID=8fd67b94-0dd5-4df0-a690-066460e47fd8 /usr
btrfs subvol=usr 0 0
UUID=8fd67b94-0dd5-4df0-a690-066460e47fd8 /boot
btrfs subvol=boot 0 0
UUID=8fd67b94-0dd5-4df0-a690-066460e47fd8 /home
btrfs subvol=home 0 0
Boot and updates with yum, dnf and pk-offline all work.
The stateful settings and files in /var, /etc, and optionally /home
can all go together on root. And stateless stuff on /usr. This seems
more coherent than a bunch of different mountpoints under var; and
even var on its own has been rather problematic.
I'm guessing the installer doesn't permit separate /usr for legacy,
pre-usrmove reasons. This layout is also compatible with the proposal
found here:
http://0pointer.net/blog/revisiting-how-we-put-together-linux-systems.html
--
Chris Murphy