This one was the solution:
- boot from microSD
- open up a terminal
- go to the folder holding your downloaded image file
- get root via
sudo su -
- run
xzcat Manjaro-ARM-sway-pbpro-20.08.img.xz | dd of=/dev/mmcblk2 bs=1M status=progress conv=fsync
to flash it to eMMC - after it’s done reboot the pbpro and follow the initial setup wizard steps