Diagnose upgrade problems

After avoiding the upgrade to Turris OS 5.x for quite some time now, I tried to tackle the task today - and failed miserably.
That is: Turris Omnia (1st Edition) with 3.11.21, upgrading to latest stable 5.x
I gave this multiple attempts. Recovering to 3.x successfully after each failure by using the previous snapshot.

Now, to be able to provide an adequate problem description, I would like to know this:

  1. Is there a way to run the upgrade from an SSH shell? I’ve trouble to follow the upgrade process from the Foris-UI. It obviously stalls or fails at some point, leaving everything in a somewhat undefined state.

  2. What is the best way to diagnose upgrade problems? Which log file to track? Which files to check?

  3. What information should be provided here, so that the problem can be reviewed?

(Sorry, if these are FAQs, but I was not able to find anything yet)