One solution is given:
Anyway, I would prefer to reboot e.g.in cooperation with a more mobile iOS device - possible?
Apple's own Configurator 2 document (https://help.apple.com/configurator/mac/2.6/#/apdebea5be51) states:
In certain circumstances, such as a power failure during a macOS upgrade, an iMac Pro may become unresponsive and must be restored. The requirements for doing an iMac Pro restore are...
Since Apple's own document makes no mention of Secure Boot being enabled or disabled, it is reasonable for people to assume that a second Mac is required for all restores, not only when Secure Boot is enabled, and not only for firmware update issues, as it specifically mentions "macOS upgrade". If anyone disagrees with Apple's own official documentation, then please contact Apple and have them update it with your superior corrections.
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