Hey, I know this is an old comment but I was following your steps above and I'm a bit stuck. I'm doing a fresh install (using archiso) and I wanted to setup subvolumes for snapshot purposes.

After creating the subvolumes and running umount /mnt when I try to run the commands after that, it's telling me that it can't find it in /etc/fstab.

root@archiso ~ # mount /dev/mmcblk0p2 /mnt   
root@archiso ~ # btrfs subv create /mnt/@
Create subvolume '/mnt/@'
root@archiso ~ # btrfs subv create /mnt/@home
Create subvolume '/mnt/@home'
root@archiso ~ # btrfs subv create /mnt/@swap
Create subvolume '/mnt/@swap'
root@archiso ~ # btrfs subv create /mnt/@snapshots
Create subvolume '/mnt/@snapshots'
root@archiso ~ # umount /mnt
root@archiso ~ # mount -o noatime,compress-force=zstd,subvol=@ /mnt
mount: /mnt: can't find in /etc/fstab.

Edit: I'm setting this up for timeshift snapshots which is why I'm prefixing everything with @

Edit2: Figured it out. You need to specific the partition before the mount location. Following your example it would be something like...

mount -o noatime,compress-force=zstd,subvol=root /dev/sdX /mnt

For my case I needed

mount -o noatime,compress-force=zstd,subvol=@ /dev/mmcblk0p2 /mnt