@Artem Komissarov
Thank you for your reply, Artem. I did try to run armbian-config several times with ext4 and all other available install options but it stalled after the initial booting with the u-boot logo and before the armbian ubuntu logo appeared. During the installation phase, it did not recognize the eMMC nor any mmcblk2 partition, but only the mmcblk1 partition where armbian-ubuntu was installed from the sd card which I flashed it to.
Although I surely must have missed some basic command somewhere during installation, it nonetheless seems that with the factory pre-installed manjaro on Pinebook Pro any attempt at installing another OS might make the eMMC unreacheable. No resetting of the PBP will have any effect, leaving booting from the sd card as the only choice.
Any further hint or advice on how to solve this issue would be very much appreciated.