Failing reinstall of El Capitan on formatted HD - no Disk Utility (2012 iMac, white screen of death)

Dear Jack

Thanks for the quick response. Your answer certainly is not what I wanted to read... ;-)

Sure, the iMac is a bit old, but it was and still could be very useful - at least, for example, as a 2nd monitor at home office times...

I had run the diagnostics which showed no problem (except for once or twice, where a memory error (4MEM/62/40000000:…) had been found (allegedly?!), but that occured only once in like 8-10 dignostics runs, and I had furthermore rearranged the 4 memory modules in various ways; no reproducibility of the alleged memory error).

What I was wondering is whether fact that neither the OS was installed from USB nor had the Disk Utility shown up with the USB stick could provide a hint to some error - either an error of the iMac or an error which I might have produced preparing the USB stick or attempting the installation of the new system... (should I rather copy the HD contents back on the iMac HD and try again then - for some reason I don't know? Would an internet connection be required during the USB-based installation?) Or the fact that the system seems to load (Apple symbol, plus status bar filling up to about 60%), and then a white screen is displayed (in the situation before formatting the HD and also now; and when attempting the USB-based installation, a black screen is quickly displayed in between).

What I also did is let the iMac start in verbose mode (command-V at startup), I am trying now (from photos I made from the screen) to see whether I can infer something useful from the many outputted lines... (which doubt...)