End state. I want to reformat my / partition, reinstall opensuse tumbleweed, and have the same packages installed then as now.
Problem. I made what appears to be a poor choice, formatting the root filesystem as btrfs. The 20 GB I had for it was plenty previously with, say, ext3, but I can't keep up with the snapshot management with snapper with the large number of updates tumbleweed does.
Partial success. I can export my repository list with
sudo zypper lr -u --export repo.list, and add that back to a fresh install with
sudo zypper addrepo repo.list. I can export a list of installed pakcages with
sudo zypper search --installed-only > installed.packages or
rpm -qa | sort.
But I don't know how to install packages from a list, or generate a list of packages that can be used by opensuse at distribution install time.
Edit: autoyast may be the way to go, but is more heavyweight than I was looking for.