Cannot boot from SD on Convergence 3 GB model

[wibble]

I wonder though if I have accidentially omitted a step after these two earlier commands

sudo bmaptool copy ~/tmp_ubuntu/ubuntu-touch-pinephone.img /dev/sdb1

xz --decompress --stdout ~/tmp_postos/20220119-0539-postmarketOS-v21.12-sxmo-de-sway-1.7.0-pine64-pinephone-installer.img.xz | sudo dd of=/dev/sdb1 bs=4M status=progress

or if there is another reason that only etcher worked (the SD card is a newly purchased Samsung evo).

Your mistake is writing the image to the first partition on the uSD card (sdb1) when you should be writing to the whole card (sdb), so the data the bootloader is looking for is in the wrong place.