Solved. This topic here should really be bookmarked.
The issue of a hard hang of the Pico as soon as a flashing should begin is also addressed by earlephilhower on his Pico Github page (see bottom part).
"In some cases the Pico will encounter a hard hang and its USB port will not respond to the auto-reset request. Should this happen, just follow the initial procedure of holding the BOOTSEL button down while plugging in the Pico to enter the ROM bootloader."
But in my case I would need to press the BOOTSEL button every time prior to USB insertion.
The topic in the first url in my post, by @Papirrruqui , is essential.
- He describes how to reset the Pico to its factory status (check this link to the official Pico website) at the bottom of that page: Resetting Flash Memory
- he also shows how to clear the USB bus connections history on the Pico on your pc.
- and finally how to proceed with the first few Pico flash operations.