Incremental Backup of BTRFS container to a sometimes plugged external device


I want to backup my server sometimes to an external device. I want to keep it incremental because I want to save backup memory. I tried to do btrfs send /var/lib/lxd/snapshots/container/snap | btrfs receive on /mnt/backup,
but it saves the .yaml files there only.

Is there a good way to do this? I don’t want to add another storage pool and copy it in the lxc way with a backup container, because I plug in the external device sometimes only.

Thank you in advance!

I tried several ways.

my last way is to create a btrfs snapshot … but if i want to copy it with btrfs send | btrfs receive
then it copies the container .yaml files only. because rootfs is an own subvolume etc.
how can I do a copy of the whole container to an external device per BTRFS?

Is this a legit way?

btrfs property set -ts server/snap-21-dec-2020/rootfs ro true

after this btrfs send for snap-21-dec-2020 and rootfs.