My PI's are again working as expected. Long story short, I deleted the pre-configuration within the RPi-Imager and everything worked fine with an SD-Card after activating the config option "hdmi_force_hotplug=1". 

I replaced the pre-configuration and everything worked fine with the SD-Card again. 

After this success, I installed the image to my earlier mentioned USB-Stick (with "hdmi_force_hotplug=1"). The first thing I saw was the message "mmc1: Controller never released inhibit bit(s) " on the screen. After 5 seconds or so, the initial boot process started. After finishing this process, my RPi performed a restart, did "something" and rebooted again. After the last boot, the RPi booted into the PiOS Desktop.

What I've learned is that **the message "mmc1: Controller never released inhibit bit(s) " seems to be irrelevant and is only seen during USB-Boot.** 
Furthermore it's new for me that the actual Bullseye Image reboots twice after initial setup. All later startups do not reboot and are quicker. 
And last but not leased during USB-Boot **it seems that there is no activity to see on the green LED while loading the Desktop**. While using an SD-Card you can see constant LED flickering until the Desktop has loaded. 

Thanks for your support.