Big Sur not installing, then safe mode: but "no administrator"

Problem: I can't access my HD with some important mails I haven't backed up.

What happened: Big Sur update (from Mojave, MBP early 2015) always froze at 35% (12h/5 trials) after going into safe mode/boot, it opened there in Big Sur, but my new user account during setup didn't turn into the administrator.

So there is now admin (also my "old" admin account from Mojave didn't work), so I am not able to delete potential stalling software nor use my new MBP 2020 Mac chip as an eternal hard drive to export the available (unlocked and mounted) Files on my HD under my original username, because it is neither admin nor user in safe mode.

I tried to fix this problem by starting in Single User Mode several times and rebooting, so that I can add a new admin account, but it didn't work, because it wouldn't recognise the commands.

Is there any other way to set up an admin account in safe mode or uninstall Big Sur so I can have access again to my usual account?

Please help.