system restart or start after idle fails / hangs after 30 % - only 'cold' start or secured mode possible

iMac (Retina 5K, 27-inch, 2017)

    iMac Model: iMac18,3

    3,4 GHz Quad-Core Intel Core i5 (i5-7500) CPU: 4-core

MacOS Mojave & Big Sur 11.4 (updated)

Restart via Apple menu - 'restart' shuts down, but hangs on re-booting after ~30 %. Screen gets black or freezes e.g. in half black - half white. No further activity, even after long waiting time. No reaction to mouse or keyboard input.

Same for waking-up from idle state. Just screen off/on works when system is running regularly.

Starting in diagnostic mode and NVRAM, PRAM and SMC reset doesn't resolve the problem.

After system hang / re-boot failure, only a restart in safe / secured mode is possible. It takes some time, but is usually possible, at least. After shutting down this safe / secured mode again, a regular restart may work.

In daily usage, a regular start / operation is only possible after complete shutdown (and maybe some additional waiting time).

Problem started some months ago under Mojave (without any conscious hard- and software change / modification), and continued to be present after updating to Big Sur. All other apps are updated as well.

Disconnecting external USB hardware (hard drives, USB hub) as sometimes recommended, also didn't solve the problem.