Add a system user for btrbk:

useradd -c "Btrbk user" -m -r -s /bin/bash -U btrbk

Setup sudo for btrbk:

echo "btrbk ALL=NOPASSWD:/usr/sbin/btrfs,/usr/bin/readlink,/usr/bin/test" | tee -a /etc/sudoers.d/btrbk

Create a subvolume for each client:

mount /dev/sda1 /mnt/storage
btrfs subvolume create client_hostname

Create a dedicated SSH key:

mkdir -p /etc/btrbk/ssh
ssh-keygen -t ed25519 -f /etc/btrbk/ssh/id_ed25519

Add each client's SSH public key to /home/btrbk/.ssh/authorized_keys:

ssh-copy-id -i /etc/btrbk/ssh/id_ed25519 btrbk@nas.local

Or, simply copy /etc/btrbk/ssh/id_ed25519.pub and paste it into /home/btrbk/.ssh/authorized_keys.

Create /etc/btrbk/btrbk.conf:

transaction_log /var/log/btrbk.log
snapshot_preserve_min latest
target_preserve 24h 7d 1m 1y
target_preserve_min 7d
ssh_user btrbk
ssh_identity /etc/btrbk/ssh/id_ed25519
backend btrfs-progs-sudo
snapshot_dir /btrbk_snapshots
target ssh://nas.local/mnt/storage/<client_hostname>
subvolume /
subvolume /home
snapshot_create ondemand

Create directory to store btrbk snapshots:

mkdir /btrbk_snapshots

Create /etc/systemd/system/btrbk.service:

[Unit]
Description=Daily btrbk backup

[Service]
Type=simple
ExecStart=/usr/bin/btrbk -q -c /etc/btrbk/btrbk.conf run

Create /etc/systemd/system/btrbk.timer:

[Unit]
Description=Daily btrbk backup

[Timer]
OnCalendar=*-*-* 23:00:00
Persistent=true

[Install]
WantedBy=timers.target

That's it.