Did you know that Automatic Updates are in the works for Drupal 9/10 as a contributed module?
The module applies patch-level updates to Drupal core in a separate, sandboxed copy of your site, to keep you up and running until the update is completely ready to be deployed. It can detect and report problems at every stage of the update process, so you don't have to find out about them after an update is live. It automatically detects database updates in an incoming update, and helps you run them during the process.
It also provides a robust API for error detection and hooking into every stage of the update process to customize it and react to update events. The API can be used to check for errors and stop the update from proceeding, or even starting in the first place, all without affecting your live Drupal site. It can also be used to implement custom triggers at each successful stage of the update process.
Acquia dedicated a strong group of developers to work on this project as part of a community team with folks from Pantheon, BioRAFT, the Drupal Association and others. Ted Bowman of the Acquia Drupal Acceleration Team presented the initiative's progress at DrupalCon Portland, the recording is worth a watch:
The project is currently at version 2.0-beta4, and works with Drupal 9.2.0+. Follow the beta testing instructions to help you get started and provide feedback to the team.