My current backup "solution". Both source and destination filesystems are btrfs. This will be replaced eventually; you should use btrfs-send or similar instead.

backup.sh

#!/usr/bin/env bash

################################

BLOCK_DEVICE=/dev/disk/by-label/Backup
DM_NAME=cryptbackup
MOUNT_POINT=/mnt/backup

################################

color() { echo -n "$(tput bold)$(tput setaf ${1})${2}$(tput sgr 0)" ; }

abort() { echo "$(color 1 'ERROR:') $(color 7 "${@}")"; exit 1 ; }

warn() { echo "$(color 3 'WARN:') $(color 7 "${@}")" ; }

info() { echo "$(color 4 'INFO:') $(color 7 "${@}")" ; }

step() { echo "$(color 4 '::') $(color 7 "${@}")" ; }

################################

set -euo pipefail

DM_PATH="/dev/mapper/${DM_NAME}"
SRC="/"
PARENT="${MOUNT_POINT}/$(hostname)"
DEST="${PARENT}/latest"
SNAPSHOT="${PARENT}/$(date --iso-8601)"

# Make sure we have root privileges
[ "$EUID" -eq 0 ] || exec sudo bash "$0" "$@"

# Make sure the backup block device exists before mounting
[[ -e "${BLOCK_DEVICE}" ]] \
    || abort "'${BLOCK_DEVICE}' is not a valid block device."

# Open block device if not already opened
if [[ ! -e "${DM_PATH}" ]]; then
    step "Opening block device '${BLOCK_DEVICE}'..."
    cryptsetup open -- "${BLOCK_DEVICE}" "${DM_NAME}" \
        || abort "Unable to open device."
fi

# Mount mapped device if not already mapped
if ! mountpoint -- "${MOUNT_POINT}" > /dev/null; then
    step "Mounting '${DM_PATH}' at '${MOUNT_POINT}'..."
    mount -o compress=zstd:2 -- "${DM_PATH}" "${MOUNT_POINT}" \
        || abort "Unable to mount device."
fi

# Make sure ${BACKUP_ROOT}/.backup exists and is a file
[[ -f "${MOUNT_POINT}/.backup" ]] \
    || abort "'${MOUNT_POINT}' missing '.backup' file."

# Check whether a backup directory exists for this host
[[ -d "${PARENT}" ]] || info "Directory '${PARENT}' will be created."
# Check whether a subvolume exists for this host
[[ -d "${DEST}" ]] || info "Subvolume '${DEST}' will be created."
# Check whether a snapshot with the same date exists for this host
[[ ! -d "${SNAPSHOT}" ]] \
    || warn "Existing snapshot '${SNAPSHOT}' will be overwritten."

# Print source, destination, and snapshot paths
echo "$(color 8 '------------------------------------------------')"
echo " $(color 4 "${SRC}") $(color 7 '->') $(color 5 "${DEST}")"
echo " $(color 0 "${SRC}") $(color 0 '  ') $(color 2 "${SNAPSHOT}")"
echo "$(color 8 '------------------------------------------------')"

# Ask for confirmation
read -p "$(color 7 'Proceed? (yes/no):') "
[[ $REPLY =~ ^(y|yes)$ ]] || exit 1

# Create parent directory if not already present
if [[ ! -d "${PARENT}" ]]; then
    step "Creating directory '${PARENT}'..."
    mkdir -- "${PARENT}" || abort "Unable to create directory."
fi

# Create destination subvolume if not already present
if [[ ! -d "${DEST}" ]]; then
    step "Creating subvolume '${DEST}'..."
    btrfs subvolume create -- "${DEST}" || abort "Unable to create subvolume."
fi

# Synchronize files (index is built first)
step "Synchronizing files..."
rsync --archive                                                               \
      --acls                                                                  \
      --xattrs                                                                \
      --sparse                                                                \
      --relative                                                              \
      --numeric-ids                                                           \
      --delete                                                                \
      --delete-excluded                                                       \
      --include='/home/***'                                                   \
      --exclude='*'                                                           \
      --info=progress2                                                        \
      -- "${SRC}" "${DEST}"
( [ $? -eq 0 ] || [ $? -eq 24 ] ) || abort "rsync exited with errors."

# Remove existing snapshot with the same date if present
if [[ -d "${SNAPSHOT}" ]]; then
    step "Removing existing snapshot '${SNAPSHOT}'..."
    btrfs subvolume delete -- "${SNAPSHOT}" \
        || abort "Unable to delete snapshot."
fi

# Create new snapshot
step "Creating snapshot '${SNAPSHOT}'..."
btrfs subvolume snapshot -r -- "${DEST}" "${SNAPSHOT}" \
    || abort "Unable to create snapshot."

# Unmount device
step "Unmounting '${MOUNT_POINT}'..."
umount -- "${MOUNT_POINT}" || abort "Unable to unmount device."

# Close device
step "Closing '${DM_PATH}'..."
cryptsetup close -- "${DM_NAME}" || abort "Unable to close device."

step "Backup complete."