I suggest trying --debugsolver with both the download and reboot subcommands. 'man dnf system-upgrade' suggests --debugsolver should work. I recommend taring the debugdata dir that will create, and setting the filename and attachment description so it's clear whether the tarball is for reboot or download.
The debugdata dir will be located in the current dir. So that'll be easy to find for the download command. But I have no idea where it'll be located for reboot so you'll probably have to use find.
Sometimes these are bigger than the 20MB attachment size, so you might have to stick it in a dropbox/googledrive like thing and post the URLs for them in the bug report.
--- Chris Murphy