U-boot needs to be patched to handle the 3GB variant of the Pinephone. Virtually all systems (except the default Postmarket, plus Manjaro, Arch, Mobian and Jumpdrive) lack the patch and will therefore fail to boot.
We do have our own build with Phosh - but you can easily add Gnome (aka Fedora workstation) or KDE-Desktop or XFCE to it,
and choose during boot (Lomiri is on the way too). Gnome actually has more features working (like auto screen rotation).
We are working on a fresh image - should be ready in a few days.
I've downloaded the .tar.xz image but should I do something else with it before performing the flash? BalenaEtcher tells me the image is missing the partition table.