Well, odd things have happened.
I was able to boot with the config.plist of
@Namok88. The system preference panel suggested I upgrade to 10.15.5 as if it had never tried it before. It then re-downloaded the update and offered to reboot to apply it.
Nevertheless: same problem. At the first reboot: no installation volume.
Then I was able to boot also with the config.plist of
@Big-W. During this test boot, my Linux volume appeared, which was not the case before. The system preference panel considered this time that it had downloaded the update correctly and directly offered to reboot to apply the update. And the problem persisted: no installation volume in Clover.
I admit that I specified the iMacPro1.1 definition and applied the patches to activate the Navi GPU.
I will try to use my CCC backup to go back to my previous setup. Not much hope. Just hope for a lucky break but I'm running out of ideas.
If it doesn't work I'll try the Big-W config.plist without adjustment.
Thank you very much for your time!