It looks like the version of Libre office you try to install cannot work together with the version of Open Office you have. Try to boot into the recovery mode and fix broken packages from there. To achieve this, choose Advanced options for Elementary OS from the grub bootmenu screen, then Recovery mode. Connect to the network from there first, and after ...
I finally got it fixed.
I had to remove the apt.llvm.org repository from the sources.list. Then I could use the Force Version command in Synaptic to switch over to using LLVM builds from the kisak PPA (which still provides i386 builds of LLVM). That enabled me to reinstall steam, because it could finally get those 32-bit i386 library builds again.
After executing sudo apt update I see this line in terminal's output:
http://packages.elementary.io/appcenter bionic InRelease
http://packages.elementary.io/appcenter juno Release
As my system doesn't have any issues with updates, you could try to swap the lines and see if it helps. (If it doesn't, just revert the process and try other ...