Although I won't be able to provide much/any logs or so, I just want to report that yesterday I had another super slow upgrade.
SG1100, from release 23.09.1 to 26.03.1 .. the downloads took over 1hr and 15 minutes.
This is a 300 MBit/s line, I had factory reset the appliance and plugged it into LAN via DHCP.
The provider is located in Austria: AS # 49163, if that is of any interest.
The active upstream gateway also was a pfSense, just mentioning (two appliances in that network, cold standby ...). Internet worked fine on other devices at that time.
A reinstallation with the installer would have been an option, but would also result in downloads, so I had decided to just wait for it to keep the overall time as short as possible.