I ran the upgrade step a couple of times now. It appears to continue for a few packages every time, but still fails with the same db error, although for different packages each time.
Additionally, I see complains about file conflicts for some packages:
file conflict found: file conflict between 'app-misc-2.0/geoclue-2.5.3-r2' and 'layers/gnome-common-0.8+11' ( file: etc/dbus-1/system.d/org.freedesktop.GeoClue2.Agent.conf )
file conflict found: file conflict between 'www-client/google-chrome-91.0.4472.114' and 'apps/google-chrome-91.0.4472.114+9' ( file: opt/google/chrome/MEIPreload/manifest.json )
I tried removing some of the old conflicting packages using equo, but even after uninstalling them, luet still complains about the same conflicts.
Furthermore, luet appears to be painfully slow. Each upgrade try takes many hours (while only processing a few packages) until it hits the db error again.
Honestly, this looks pretty much broken. Would doing a fresh install yield a more useable system?