So after about 20 unsuccessful attempts, including another USB stick, buying another NVMe drive, and various disk partitioning approaches (fully automatic, custom with recommended settings, or doing all the partitioning from a live system and just using the existing partitions in the installer), I successfully installed Qubes 4.1.2 yesterday. The solution seems to be dom0_max_vcpus=1 – and only that. Another thread recommending the 14" version for the HCL mentioned that dom0_max_vcpus=1 dom0_vcpu_pin are necessary to get the installer running, but the second option is not recognized and causes the installer to quit immediately in my case.
So in the end, it seems everything that was needed is hitting e after selecting Install Qubes OR R4.1.2 with the cursor keys, then replacing quiet by dom0_max_vcpus=1 and hitting F10. The installer then completes without problems.
I guess there is an issue in anaconda with regard to multithreading, so another thread tries to check if the logical volume creation or formatting has finished before it actually started, raising a Python exception. 
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