FTW.. to get into Fastboot Mode via buttons is really difficult. But after several trys with different timings it worked like described:
"Then pick your clacky Pro1x, power it down, hold down [Volume -] and [Power] until it boots. It will show a dead droid logo. From there, hold [Power] and without releasing it, hit [Volume +] briefly with another finger. From the menu that just appeared, select Bootloader. The phone will reboot into bootloader mode, where you can use fastboot commands"
I'm back with DROIDIAN. 🤓