Just for the future, seeing an M-600 take an extended amount of time to perform major version updates is expected behavior. During the upgrade from 9.1 to 10.0 every single log file on the M-600 has to be migrated, so the more logs a system has the longer the process is going to take. I've seen some updates take upwards of 5 hours. Maintenance updates are much quicker because you don't have to do anything to migrate the logs, as they are all already in the proper format.
Also I'd also put out that I personally wouldn't recommend upgrading your M-600 (presumably functioning in Panorama mode) and your firewalls at the same time. If you run into a bug or issue post upgrade and need to downgrade your M-600 you would also need to downgrade your PA-5220s at the same time so they can be managed by Panorama.
I would recommend upgrading Panorama the week prior to upgrading any of your firewalls. This allows you enough time to see if you'll run into any issues on the Panorama upgrade prior to upgrading any of the firewalls, so you don't have the risk of needing to downgrade multiple devices at the same time.