Can I just use dd to clone my laptop to a bootable USB stick?
I know about Clonezilla and copy pasting partitions with gparted, but can I just use dd to copy a partition with batocera to a USB stick and will it then boot from the stick? Do I have to set the boot flag or take any other steps?
Yes, but like @kuneho said, since "deleted" stuff only is marked as deleted (not wiped), there's always a bunch of random on the "empty" space part of a disk, which compresses badly.
Do cat /dev/zero > /path/to/mounted/partition/zeroes and delete it after cat errored out because no space, to fill the "empty" space with zeroes.
That worked really well! I got a 50gb partition with about 30gb free space into a 10gb zipped image. Is there any way to show progress during the operation like with dd's status=progress?
I often use pv instead of cat for this. And there's some 'hack' where a specific tool looks at some kernel feature to guess progress of cp & co. But i forgot it's name.