Failed to upgrade Big sur11.2.3 to 11.3.1

Hi,

I have a question.

For some time now I don't get any upgrade notification through the standard app in system preferences. I didn't find any real cause for that but i've read i'm not the only one and the workaround was to download the upgrade from the appstore.

I dit this also with 11.3.1 but got stuck on the first reboot. It chooses a macintosh HD as the boot drive instead of the drive where i choose to install it from.

This seems to be an read only drive that contains some volumes even EFI is present (but empty).

When I reboot and choose my normal Macos drive. It rolls back the update.

I uninstalled ESET and tried again same result.

I waited at the apple logo screen for hal an hour.

After that the macintosh HD is still in my boot options Even after formatting the EFI partition and putting a new EFI folder. Just deleting some Apple folder which is normally not there did not work als and I have some volume called update should I remove it?

Resetting NVRAM does not change anything. My startup disk in system preferences is the right (only) one.

Is there a way to get passed this? I have ImacPro7,1 in my smBios. When I go to recovery I get the choice to reinstall is that an option without loosing my data? (there is a lot of tweaking I dont want to loose). I can go back with time machine, but 900GB restore is my last resort.

Any tips?