That would have been the last resort but it would also mean having to reinstall everything/restoring from TimeMachine.
I am not completely sure, but I think I remember that it was because of this error that I switched from USB booting to launching the installer from Mac OS. Had not experienced this issue again until now.
Most of my Macs are also dualboot systems, but this is the only iMac which in addition to Windows 11 also has Linux Mint Cinnamon installed (which runs really well btw).
Today, I took the risk and updated my other machines to 15.7.2 final (all were on OCLP 2.4.1 already):
iMac 2011 27", i7 2600, 1TB SSD, 24GB RAM, K2100m
iMac 2011 21,5", i7 2600s, 1TB SSD, 20GB RAM, K2100m
MBP 2015, 13", i5, 8GB RAM, 256MB stock SSD
Because of my bad experience with USB booting the installer, this time, I started it from the existing mac OS installations. Used the same USB stick, however, than the last time. Everything went well and the results seem to be smoothly working machines. Closely monitoring the installation processes, I noticed the installer proceeds differently than when USB booted. I have the impression that more is done within the existing macOS GUI. The "mac OS installer" entry in OC appeared only with the first reboot, afterwards "Macintosh HD" was the only availably option in the picker. The dreaded black screen in the end did not count down from something like 58 min (or so) but only 8 mins.
I would hence suggest, if one already has a working installation, to start the update from within it. Positive side effect: The same USB stick can be used for different machines.