extract_from_diskimg.sh

#!/bin/bash
set -x

dump_file=$1

if [[ -z "$dump_file" ]]; then
	echo "No dump provided assuming \"mmcblk0\""
	dump_file="mmcblk0"
fi

line=$(sfdisk -d $dump_file | grep "start" | awk -F'[, ]*' '{print "dd if='$dump_file' of=" substr($9, 7, length($9)-7) ".img skip=" $4 " count=" $6}')
while IFS= read -r l; do
	$l
done <<< "$line"

Use the command sfdisk -d mmcblk0 > partition_mapping to get a partition map.
Get the entire disk image by using adb pull /dev/mmcblk0 ..

Another way to get the image using ssh access is to execute this on the host Linux computer:

ssh root@watch "dd bs=1M if=/dev/mmcblk0" | dd of=mmcblk0_b.img bs=1M

This assumes that watch has been set up with the IP address of the watch. This works over WiFi, but of course, transferring 4G via WiFi is going to be relatively slow.