Question: how to shrink qcow2 image?
Answer: Shut down the virtual machine.
Resize the image with qemu-img resize image.qcow2 +SIZE. where SIZE is the size (e.g. 10G for 10 gibibytes).
Boot into an external live OS and resize the partition. The easiest way to do this is to use a GParted live image and virt-manager to connect to the VM.