Problems with faulty update installer boot
From AppleToolBox.com:


  Fixing macOS Install Problems in Recovery Mode
  
  To get to recovery mode on your Mac or MacBook, you will need to press and hold Command+R keys together when booting up your computer.
  
  Once you are in the Recovery mode, click on the Apple logo on the top left corner of the screen and make sure that your startup disk is pointing to the Mac Boot disk.
  
  Now find the macOS update program and run the update. Sometimes, when you are facing install/upgrade issues, a workaround that works for many users is to create an external bootable installer and try to run the install program using it. Apple provides a detailed white paper on this process. You can create the bootable installer on a USB and then try it to update the macOS.