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.
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Chris Murphy