I do not think the “failed to start SDDM” issue is an issue with the sddm package, but a transitive dependency issue, most likely this one:
which is due to the installation having qt5-es2-* packages still installed. Those are no longer maintained and depend on old libraries, which makes SDDM fail to start. The fix:
sudo pacman -Sy qt5-base qt5-declarative qt5-multimedia pyside2
and saying yes to replacing the es2 equivalents.
The issue affects both the PinePhone and the PinePhone Pro, because the profiles for both of those used to install the qt5-es2-* packages by default.