I just did 2 installs from the original 2018.3.0, one on bare metal and one in KVM/QEMU and both times hit this problem with appstream. Both times purging it from live allowed the installer to complete happily. Hopefully this is addressed before the next release.
I was most impressed that doing d-u of over 1900 packages worked without a hitch! I will remember to ask on IRC for a more recent image next time if it's a long time since release, which may be soon as I have one more install to do soon.
Like the other poster here, I was also tempted to change to Debian stable and installed Buster with the intention of tracking it to release and then sticking with it. siduction is just
better though so Buster hasn't won me over!
The only issue I've had with upgrades for a very, very long time is with Nvidia driver for the new 5.0 kernel and even that was easy to fix with the nvidia-driver package from experimental.