The issue was caused by the jonathonf/python-3.6 PPA I had enabled in 16.04. The upgrade to 18.04. disabled this PPA, installed the libpython3.6 in the non-ppa version (the york version are from the PPA), and therefore caused a missing dependency. First I had to revert the downgrade mentioned in the Edit with analogous apt download and forced dpkg commands.
I reenabled the PPA (also changed bionic to xenial in /etc/apt/sources.list.d/jonathonf-ubuntu-python-3_6-xenial.list) and after an easy
sudo apt update && sudo apt install -f && sudo apt upgradeeverything is working fine now again.