macos installer finishes update installation to sonoma but doesn't show it in About this Mac

That's a new one, what does it show in About this Mac, a screen shot would be ideal! Also, have you attempted any troubleshooting such as restarting normally or restarting in Safe Mode with the keyboard's charging cable connected??

This could potentially indicate a glitch or hiccup in the installation process that prevents the system from properly updating the About This Mac section to reflect the new version of macOS. It's unusual for the installation to complete without errors but not reflect the update in the system information.

One step you could try is to restart your iMac after the installation completes. Sometimes, a simple restart can help refresh the system information and ensure that the update is properly recognized. After restarting, check the About This Mac section again to see if it now reflects the Sonoma update.

If restarting doesn't resolve the issue, you may want to try resetting the System Management Controller (SMC) on your iMac. The SMC controls various low-level functions of the computer, and resetting it can sometimes resolve unusual behavior or glitches. You can reset the SMC by following the instructions provided by Apple for your specific iMac model.

If the issue persists even after restarting and resetting the SMC, it's possible that there may be a more complex underlying issue with the installation process or your iMac's hardware. In this case, reaching out to Official Apple Support or visiting an Apple Store for further assistance would be advisable.

What are you running/doing to install it?

You might try going in stages from 11->12->13->14.