Thank you, that was it Indeed!
I took the separate json firmware file from https://src.fedoraproject.org/fork/kraxel/rpms/edk2/c/5146a0c3e9bf821d045e0cc3600ad715aca14588 (which was linked in the patch thread) and adapted the path to the OVMF file to my system. then fixed my virt-install invocation slightly (mainly removing loader.stateless and loader.type) and it works now.
