After having watched some YouTube videos about the flash-modding process, it ***''seems''*** to be normal that the iPod has an unlit screen after the restore process (and when otherwise connected to the PC/Mac with USB.. Maybe the old HDD in my iPod mini had some kind of cache for settings? In the case of the YouTubers, they may not have noticed that the screen was dim as they could easily switch on the backlight with the menu button.
https://www.youtube.com/watch?v=Fy3iNZJzc2w
https://www.youtube.com/watch?v=VC-tSuapbKQ&t=467s
That's the theory. Boy, has this otherwise cheap iPod Mini cost my me nerves (in terms of upgrading all the parts).